DATAMONGO-1564 - Split up AggregationExpressions.

Refactored to multiple smaller Aggregation Operator classes reflecting the grouping (array operators, string operators,…) predefined by MongoDB.

Original pull request: #429.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java

@@ -0,0 +1,149 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.util.ObjectUtils;
+
+/**
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+abstract class AbstractAggregationExpression implements AggregationExpression {
+
+	private final Object value;
+
+	protected AbstractAggregationExpression(Object value) {
+		this.value = value;
+	}
+
+	/* (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public Document toDocument(AggregationOperationContext context) {
+		return toDocument(this.value, context);
+	}
+
+	public Document toDocument(Object value, AggregationOperationContext context) {
+
+		Object valueToUse;
+		if (value instanceof List) {
+
+			List<Object> arguments = (List<Object>) value;
+			List<Object> args = new ArrayList<Object>(arguments.size());
+
+			for (Object val : arguments) {
+				args.add(unpack(val, context));
+			}
+			valueToUse = args;
+		} else if (value instanceof java.util.Map) {
+
+			Document dbo = new Document();
+			for (java.util.Map.Entry<String, Object> entry : ((java.util.Map<String, Object>) value).entrySet()) {
+				dbo.put(entry.getKey(), unpack(entry.getValue(), context));
+			}
+			valueToUse = dbo;
+		} else {
+			valueToUse = unpack(value, context);
+		}
+
+		return new Document(getMongoMethod(), valueToUse);
+	}
+
+	protected static List<Field> asFields(String... fieldRefs) {
+
+		if (ObjectUtils.isEmpty(fieldRefs)) {
+			return Collections.emptyList();
+		}
+
+		return Fields.fields(fieldRefs).asList();
+	}
+
+	private Object unpack(Object value, AggregationOperationContext context) {
+
+		if (value instanceof AggregationExpression) {
+			return ((AggregationExpression) value).toDocument(context);
+		}
+
+		if (value instanceof Field) {
+			return context.getReference((Field) value).toString();
+		}
+
+		if (value instanceof List) {
+
+			List<Object> sourceList = (List<Object>) value;
+			List<Object> mappedList = new ArrayList<Object>(sourceList.size());
+
+			for (Object item : sourceList) {
+				mappedList.add(unpack(item, context));
+			}
+			return mappedList;
+		}
+
+		return value;
+	}
+
+	protected List<Object> append(Object value) {
+
+		if (this.value instanceof List) {
+
+			List<Object> clone = new ArrayList<Object>((List) this.value);
+
+			if (value instanceof List) {
+				for (Object val : (List) value) {
+					clone.add(val);
+				}
+			} else {
+				clone.add(value);
+			}
+			return clone;
+		}
+
+		return Arrays.asList(this.value, value);
+	}
+
+	protected java.util.Map<String, Object> append(String key, Object value) {
+
+		if (!(this.value instanceof java.util.Map)) {
+			throw new IllegalArgumentException("o_O");
+		}
+		java.util.Map<String, Object> clone = new LinkedHashMap<String, Object>(
+				(java.util.Map<String, Object>) this.value);
+		clone.put(key, value);
+		return clone;
+
+	}
+
+	protected List<Object> values() {
+
+		if (value instanceof List) {
+			return new ArrayList<Object>((List) value);
+		}
+		if (value instanceof java.util.Map) {
+			return new ArrayList<Object>(((java.util.Map) value).values());
+		}
+		return new ArrayList<Object>(Arrays.asList(value));
+	}
+
+	protected abstract String getMongoMethod();
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AccumulatorOperators.java

@@ -0,0 +1,641 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal accumulator} aggregation operations.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ * @soundtrack Rage Against The Machine - Killing In The Name
+ */
+public class AccumulatorOperators {
+
+	/**
+	 * Take the numeric value referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static AccumulatorOperatorFactory valueOf(String fieldReference) {
+		return new AccumulatorOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the numeric value referenced resulting from given {@link AggregationExpression}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static AccumulatorOperatorFactory valueOf(AggregationExpression expression) {
+		return new AccumulatorOperatorFactory(expression);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class AccumulatorOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link AccumulatorOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public AccumulatorOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link ArrayOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public AccumulatorOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and calculates and
+		 * returns the sum.
+		 *
+		 * @return
+		 */
+		public Sum sum() {
+			return usesFieldRef() ? Sum.sumOf(fieldReference) : Sum.sumOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and returns the
+		 * average value.
+		 *
+		 * @return
+		 */
+		public Avg avg() {
+			return usesFieldRef() ? Avg.avgOf(fieldReference) : Avg.avgOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and returns the
+		 * maximum value.
+		 *
+		 * @return
+		 */
+		public Max max() {
+			return usesFieldRef() ? Max.maxOf(fieldReference) : Max.maxOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and returns the
+		 * minimum value.
+		 *
+		 * @return
+		 */
+		public Min min() {
+			return usesFieldRef() ? Min.minOf(fieldReference) : Min.minOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and calculates the
+		 * population standard deviation of the input values.
+		 *
+		 * @return
+		 */
+		public StdDevPop stdDevPop() {
+			return usesFieldRef() ? StdDevPop.stdDevPopOf(fieldReference) : StdDevPop.stdDevPopOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes the associated numeric value expression and calculates the
+		 * sample standard deviation of the input values.
+		 *
+		 * @return
+		 */
+		public StdDevSamp stdDevSamp() {
+			return usesFieldRef() ? StdDevSamp.stdDevSampOf(fieldReference) : StdDevSamp.stdDevSampOf(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return fieldReference != null;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $sum}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Sum extends AbstractAggregationExpression {
+
+		private Sum(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$sum";
+		}
+
+		/**
+		 * Creates new {@link Sum}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Sum sumOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Sum(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Sum}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Sum sumOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Sum(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Sum} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Sum and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Sum(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Sum} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Sum and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Sum(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $avg}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Avg extends AbstractAggregationExpression {
+
+		private Avg(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$avg";
+		}
+
+		/**
+		 * Creates new {@link Avg}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Avg avgOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Avg(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Avg}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Avg avgOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Avg(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Avg} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Avg and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Avg(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Avg} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Avg and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Avg(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $max}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Max extends AbstractAggregationExpression {
+
+		private Max(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$max";
+		}
+
+		/**
+		 * Creates new {@link Max}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Max maxOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Max(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Max}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Max maxOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Max(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Max} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Max and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Max(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Max} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Max and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Max(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $min}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Min extends AbstractAggregationExpression {
+
+		private Min(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$min";
+		}
+
+		/**
+		 * Creates new {@link Min}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Min minOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Min(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Min}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Min minOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Min(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Min} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Min and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Min(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Min} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Min and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Min(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $stdDevPop}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class StdDevPop extends AbstractAggregationExpression {
+
+		private StdDevPop(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$stdDevPop";
+		}
+
+		/**
+		 * Creates new {@link StdDevPop}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static StdDevPop stdDevPopOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new StdDevPop(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link StdDevPop} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static StdDevPop stdDevPopOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new StdDevPop(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link StdDevPop} with all previously added arguments appending the given one. <br/>
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public StdDevPop and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new StdDevPop(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link StdDevSamp} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public StdDevPop and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new StdDevPop(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $stdDevSamp}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class StdDevSamp extends AbstractAggregationExpression {
+
+		private StdDevSamp(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$stdDevSamp";
+		}
+
+		/**
+		 * Creates new {@link StdDevSamp}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static StdDevSamp stdDevSampOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new StdDevSamp(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link StdDevSamp}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static StdDevSamp stdDevSampOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new StdDevSamp(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link StdDevSamp} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public StdDevSamp and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new StdDevSamp(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link StdDevSamp} with all previously added arguments appending the given one. <br />
+		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public StdDevSamp and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new StdDevSamp(append(expression));
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+
+			if (value instanceof List) {
+				if (((List) value).size() == 1) {
+					return super.toDocument(((List<Object>) value).iterator().next(), context);
+				}
+			}
+
+			return super.toDocument(value, context);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOperationRenderer.java

@@ -26,7 +26,7 @@ import org.springframework.data.mongodb.core.aggregation.Fields.AggregationField
 import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
 
 /**
- * Rendering support for {@link AggregationOperation} into a {@link List} of {@link com.mongodb.Document}.
+ * Rendering support for {@link AggregationOperation} into a {@link List} of {@link org.bson.Document}.
  *
  * @author Mark Paluch
  * @author Christoph Strobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationSpELExpression.java

@@ -64,7 +64,7 @@ public class AggregationSpELExpression implements AggregationExpression {
 	}
 
 	/* (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDbObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BooleanOperators.java

@@ -0,0 +1,353 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal boolean expressions} that evaluate their argument expressions as booleans and return a boolean
+ * as the result.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+public class BooleanOperators {
+
+	/**
+	 * Take the array referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static BooleanOperatorFactory valueOf(String fieldReference) {
+		return new BooleanOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the value resulting of the given {@link AggregationExpression}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static BooleanOperatorFactory valueOf(AggregationExpression fieldReference) {
+		return new BooleanOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpressions} that evaluates the boolean value of the referenced field and returns the
+	 * opposite boolean value.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static Not not(String fieldReference) {
+		return Not.not(fieldReference);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpressions} that evaluates the boolean value of {@link AggregationExpression} result
+	 * and returns the opposite boolean value.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static Not not(AggregationExpression expression) {
+		return Not.not(expression);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class BooleanOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link BooleanOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public BooleanOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link BooleanOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public BooleanOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates one or more expressions and returns {@literal true} if
+		 * all of the expressions are {@literal true}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public And and(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return createAnd().andExpression(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates one or more expressions and returns {@literal true} if
+		 * all of the expressions are {@literal true}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public And and(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return createAnd().andField(fieldReference);
+		}
+
+		private And createAnd() {
+			return usesFieldRef() ? And.and(Fields.field(fieldReference)) : And.and(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates one or more expressions and returns {@literal true} if
+		 * any of the expressions are {@literal true}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Or or(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return createOr().orExpression(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates one or more expressions and returns {@literal true} if
+		 * any of the expressions are {@literal true}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Or or(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return createOr().orField(fieldReference);
+		}
+
+		private Or createOr() {
+			return usesFieldRef() ? Or.or(Fields.field(fieldReference)) : Or.or(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that evaluates a boolean and returns the opposite boolean value.
+		 *
+		 * @return
+		 */
+		public Not not() {
+			return usesFieldRef() ? Not.not(fieldReference) : Not.not(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return this.fieldReference != null;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $and}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class And extends AbstractAggregationExpression {
+
+		private And(List<?> values) {
+			super(values);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$and";
+		}
+
+		/**
+		 * Creates new {@link And} that evaluates one or more expressions and returns {@literal true} if all of the
+		 * expressions are {@literal true}.
+		 *
+		 * @param expressions
+		 * @return
+		 */
+		public static And and(Object... expressions) {
+			return new And(Arrays.asList(expressions));
+		}
+
+		/**
+		 * Creates new {@link And} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public And andExpression(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new And(append(expression));
+		}
+
+		/**
+		 * Creates new {@link And} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public And andField(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new And(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link And} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public And andValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new And(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $or}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Or extends AbstractAggregationExpression {
+
+		private Or(List<?> values) {
+			super(values);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$or";
+		}
+
+		/**
+		 * Creates new {@link Or} that evaluates one or more expressions and returns {@literal true} if any of the
+		 * expressions are {@literal true}.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public static Or or(Object... expressions) {
+
+			Assert.notNull(expressions, "Expressions must not be null!");
+			return new Or(Arrays.asList(expressions));
+		}
+
+		/**
+		 * Creates new {@link Or} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Or orExpression(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Or(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Or} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Or orField(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Or(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Or} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Or orValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Or(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $not}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Not extends AbstractAggregationExpression {
+
+		private Not(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$not";
+		}
+
+		/**
+		 * Creates new {@link Not} that evaluates the boolean value of the referenced field and returns the opposite boolean
+		 * value.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Not not(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Not(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Not} that evaluates the resulting boolean value of the given {@link AggregationExpression} and
+		 * returns the opposite boolean value.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Not not(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Not(Collections.singletonList(expression));
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ComparisonOperators.java

@@ -0,0 +1,879 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal comparison expressions}.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+public class ComparisonOperators {
+
+	/**
+	 * Take the field referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static ComparisonOperatorFactory valueOf(String fieldReference) {
+		return new ComparisonOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the value resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static ComparisonOperatorFactory valueOf(AggregationExpression expression) {
+		return new ComparisonOperatorFactory(expression);
+	}
+
+	public static class ComparisonOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link ComparisonOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public ComparisonOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link ComparisonOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public ComparisonOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareTo(String fieldReference) {
+			return createCmp().compareTo(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareTo(AggregationExpression expression) {
+			return createCmp().compareTo(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareToValue(Object value) {
+			return createCmp().compareToValue(value);
+		}
+
+		private Cmp createCmp() {
+			return usesFieldRef() ? Cmp.valueOf(fieldReference) : Cmp.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is equal to the value of the referenced field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalTo(String fieldReference) {
+			return createEq().equalTo(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is equal to the expression result.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalTo(AggregationExpression expression) {
+			return createEq().equalTo(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is equal to the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalToValue(Object value) {
+			return createEq().equalToValue(value);
+		}
+
+		private Eq createEq() {
+			return usesFieldRef() ? Eq.valueOf(fieldReference) : Eq.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than the value of the referenced field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThan(String fieldReference) {
+			return createGt().greaterThan(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than the expression result.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThan(AggregationExpression expression) {
+			return createGt().greaterThan(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThanValue(Object value) {
+			return createGt().greaterThanValue(value);
+		}
+
+		private Gt createGt() {
+			return usesFieldRef() ? Gt.valueOf(fieldReference) : Gt.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than or equivalent to the value of the referenced field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualTo(String fieldReference) {
+			return createGte().greaterThanEqualTo(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than or equivalent to the expression result.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualTo(AggregationExpression expression) {
+			return createGte().greaterThanEqualTo(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is greater than or equivalent to the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualToValue(Object value) {
+			return createGte().greaterThanEqualToValue(value);
+		}
+
+		private Gte createGte() {
+			return usesFieldRef() ? Gte.valueOf(fieldReference) : Gte.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than the value of the referenced field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThan(String fieldReference) {
+			return createLt().lessThan(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than the expression result.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThan(AggregationExpression expression) {
+			return createLt().lessThan(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than to the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThanValue(Object value) {
+			return createLt().lessThanValue(value);
+		}
+
+		private Lt createLt() {
+			return usesFieldRef() ? Lt.valueOf(fieldReference) : Lt.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than or equivalent to the value of the referenced field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Lte lessThanEqualTo(String fieldReference) {
+			return createLte().lessThanEqualTo(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than or equivalent to the expression result.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Lte lessThanEqualTo(AggregationExpression expression) {
+			return createLte().lessThanEqualTo(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the first
+		 * value is less than or equivalent to the given value.
+		 *
+		 * @param value
+		 * @return
+		 */
+		public Lte lessThanEqualToValue(Object value) {
+			return createLte().lessThanEqualToValue(value);
+		}
+
+		private Lte createLte() {
+			return usesFieldRef() ? Lte.valueOf(fieldReference) : Lte.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the values
+		 * are not equivalent.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualTo(String fieldReference) {
+			return createNe().notEqualTo(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the values
+		 * are not equivalent.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualTo(AggregationExpression expression) {
+			return createNe().notEqualTo(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares two values and returns {@literal true} when the values
+		 * are not equivalent.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualToValue(Object value) {
+			return createNe().notEqualToValue(value);
+		}
+
+		private Ne createNe() {
+			return usesFieldRef() ? Ne.valueOf(fieldReference) : Ne.valueOf(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return fieldReference != null;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $cmp}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Cmp extends AbstractAggregationExpression {
+
+		private Cmp(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$cmp";
+		}
+
+		/**
+		 * Creates new {@link Cmp}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Cmp valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Cmp(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Cmp}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Cmp valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Cmp(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareTo(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Cmp(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Cmp(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Cmp compareToValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Cmp(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $eq}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Eq extends AbstractAggregationExpression {
+
+		private Eq(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$eq";
+		}
+
+		/**
+		 * Creates new {@link Eq}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Eq valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Eq(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Eq}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Eq valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Eq(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Eq} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalTo(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Eq(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Eq} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Eq(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Eq} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Eq equalToValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Eq(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $gt}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Gt extends AbstractAggregationExpression {
+
+		private Gt(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$gt";
+		}
+
+		/**
+		 * Creates new {@link Gt}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Gt valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Gt(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Gt}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Gt valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Gt(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Gt} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThan(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Gt(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Gt} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThan(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Gt(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Gt} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Gt greaterThanValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Gt(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $lt}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Lt extends AbstractAggregationExpression {
+
+		private Lt(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$lt";
+		}
+
+		/**
+		 * Creates new {@link Lt}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Lt valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Lt(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Lt}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Lt valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Lt(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Lt} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThan(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Lt(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Lt} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThan(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Lt(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Lt} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Lt lessThanValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Lt(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $gte}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Gte extends AbstractAggregationExpression {
+
+		private Gte(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$gte";
+		}
+
+		/**
+		 * Creates new {@link Gte}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Gte valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Gte(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Gte}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Gte valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Gte(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Gte} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualTo(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Gte(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Gte} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Gte(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Gte} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Gte greaterThanEqualToValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Gte(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $lte}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Lte extends AbstractAggregationExpression {
+
+		private Lte(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$lte";
+		}
+
+		/**
+		 * Creates new {@link Lte}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Lte valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Lte(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Lte}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Lte valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Lte(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Lte} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Lte lessThanEqualTo(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Lte(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Lte} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Lte lessThanEqualTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Lte(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Lte} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Lte lessThanEqualToValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Lte(append(value));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $ne}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Ne extends AbstractAggregationExpression {
+
+		private Ne(List<?> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$ne";
+		}
+
+		/**
+		 * Creates new {@link Ne}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Ne valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Ne(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Ne}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Ne valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Ne(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link Ne} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualTo(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Ne(append(Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link Ne} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Ne(append(expression));
+		}
+
+		/**
+		 * Creates new {@link Eq} with all previously added arguments appending the given one.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public Ne notEqualToValue(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Ne(append(value));
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ConditionalOperators.java

@@ -0,0 +1,973 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.dao.InvalidDataAccessApiUsageException;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond.OtherwiseBuilder;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond.ThenBuilder;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Switch.CaseOperator;
+import org.springframework.data.mongodb.core.query.CriteriaDefinition;
+import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
+
+/**
+ * Gateway to {@literal conditional expressions} that evaluate their argument expressions as booleans to a value.
+ *
+ * @author Mark Paluch
+ */
+public class ConditionalOperators {
+
+	/**
+	 * Take the field referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static ConditionalOperatorFactory when(String fieldReference) {
+		return new ConditionalOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the value resulting from the given {@literal expression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static ConditionalOperatorFactory when(AggregationExpression expression) {
+		return new ConditionalOperatorFactory(expression);
+	}
+
+	/**
+	 * Take the value resulting from the given {@literal criteriaDefinition}.
+	 *
+	 * @param criteriaDefinition must not be {@literal null}.
+	 * @return
+	 */
+	public static ConditionalOperatorFactory when(CriteriaDefinition criteriaDefinition) {
+		return new ConditionalOperatorFactory(criteriaDefinition);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpressions} that evaluates an expression and returns the value of the expression if
+	 * the expression evaluates to a non-null value. If the expression evaluates to a {@literal null} value, including
+	 * instances of undefined values or missing fields, returns the value of the replacement expression.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static IfNull.ThenBuilder ifNull(String fieldReference) {
+
+		Assert.notNull(fieldReference, "FieldReference must not be null!");
+		return IfNull.ifNull(fieldReference);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpressions} that evaluates an expression and returns the value of the expression if
+	 * the expression evaluates to a non-null value. If the expression evaluates to a {@literal null} value, including
+	 * instances of undefined values or missing fields, returns the value of the replacement expression.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static IfNull.ThenBuilder ifNull(AggregationExpression expression) {
+
+		Assert.notNull(expression, "Expression must not be null!");
+		return IfNull.ifNull(expression);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpression} that evaluates a series of {@link CaseOperator} expressions. When it
+	 * finds an expression which evaluates to {@literal true}, {@code $switch} executes a specified expression and breaks
+	 * out of the control flow.
+	 *
+	 * @param conditions must not be {@literal null}.
+	 * @return
+	 */
+	public static Switch switchCases(CaseOperator... conditions) {
+		return Switch.switchCases(conditions);
+	}
+
+	/**
+	 * Creates new {@link AggregationExpression} that evaluates a series of {@link CaseOperator} expressions. When it
+	 * finds an expression which evaluates to {@literal true}, {@code $switch} executes a specified expression and breaks
+	 * out of the control flow.
+	 *
+	 * @param conditions must not be {@literal null}.
+	 * @return
+	 */
+	public static Switch switchCases(List<CaseOperator> conditions) {
+		return Switch.switchCases(conditions);
+	}
+
+	public static class ConditionalOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+		private final CriteriaDefinition criteriaDefinition;
+
+		/**
+		 * Creates new {@link ConditionalOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public ConditionalOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			this.fieldReference = fieldReference;
+			this.expression = null;
+			this.criteriaDefinition = null;
+		}
+
+		/**
+		 * Creates new {@link ConditionalOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public ConditionalOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			this.fieldReference = null;
+			this.expression = expression;
+			this.criteriaDefinition = null;
+		}
+
+		/**
+		 * Creates new {@link ConditionalOperatorFactory} for given {@link CriteriaDefinition}.
+		 *
+		 * @param criteriaDefinition must not be {@literal null}.
+		 */
+		public ConditionalOperatorFactory(CriteriaDefinition criteriaDefinition) {
+
+			Assert.notNull(criteriaDefinition, "CriteriaDefinition must not be null!");
+
+			this.fieldReference = null;
+			this.expression = null;
+			this.criteriaDefinition = criteriaDefinition;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that evaluates a boolean expression to return one of the two specified
+		 * return expressions.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public OtherwiseBuilder then(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return createThenBuilder().then(value);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates a boolean expression to return one of the two specified
+		 * return expressions.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public OtherwiseBuilder thenValueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return createThenBuilder().then(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that evaluates a boolean expression to return one of the two specified
+		 * return expressions.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public OtherwiseBuilder thenValueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return createThenBuilder().then(fieldReference);
+		}
+
+		private ThenBuilder createThenBuilder() {
+
+			if (usesFieldRef()) {
+				return Cond.newBuilder().when(fieldReference);
+			}
+
+			return usesCriteriaDefinition() ? Cond.newBuilder().when(criteriaDefinition) : Cond.newBuilder().when(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return this.fieldReference != null;
+		}
+
+		private boolean usesCriteriaDefinition() {
+			return this.criteriaDefinition != null;
+		}
+	}
+
+	/**
+	 * Encapsulates the aggregation framework {@code $ifNull} operator. Replacement values can be either {@link Field
+	 * field references}, {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be
+	 * converted to a simple MongoDB type.
+	 *
+	 * @see http://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/
+	 * @author Mark Paluch
+	 */
+	public static class IfNull implements AggregationExpression {
+
+		private final Object condition;
+		private final Object value;
+
+		private IfNull(Object condition, Object value) {
+
+			this.condition = condition;
+			this.value = value;
+		}
+
+		/**
+		 * Creates new {@link IfNull}.
+		 *
+		 * @param fieldReference the field to check for a {@literal null} value, field reference must not be {@literal null}
+		 *          .
+		 * @return
+		 */
+		public static ThenBuilder ifNull(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new IfNullOperatorBuilder().ifNull(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link IfNull}.
+		 *
+		 * @param expression the expression to check for a {@literal null} value, field reference must not be
+		 *          {@literal null}.
+		 * @return
+		 */
+		public static ThenBuilder ifNull(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new IfNullOperatorBuilder().ifNull(expression);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+
+			List<Object> list = new ArrayList<Object>();
+
+			if (condition instanceof Field) {
+				list.add(context.getReference((Field) condition).toString());
+			} else if (condition instanceof AggregationExpression) {
+				list.add(((AggregationExpression) condition).toDocument(context));
+			} else {
+				list.add(condition);
+			}
+
+			list.add(resolve(value, context));
+
+			return new Document("$ifNull", list);
+		}
+
+		private Object resolve(Object value, AggregationOperationContext context) {
+
+			if (value instanceof Field) {
+				return context.getReference((Field) value).toString();
+			} else if (value instanceof AggregationExpression) {
+				return ((AggregationExpression) value).toDocument(context);
+			} else if (value instanceof Document) {
+				return value;
+			}
+
+			return context.getMappedObject(new Document("$set", value)).get("$set");
+		}
+
+		/**
+		 * @author Mark Paluch
+		 */
+		public static interface IfNullBuilder {
+
+			/**
+			 * @param fieldReference the field to check for a {@literal null} value, field reference must not be
+			 *          {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder ifNull(String fieldReference);
+
+			/**
+			 * @param expression the expression to check for a {@literal null} value, field name must not be {@literal null}
+			 *          or empty.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder ifNull(AggregationExpression expression);
+		}
+
+		/**
+		 * @author Mark Paluch
+		 */
+		public static interface ThenBuilder {
+
+			/**
+			 * @param value the value to be used if the {@code $ifNull} condition evaluates {@literal true}. Can be a
+			 *          {@link Document}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
+			 *          representation but must not be {@literal null}.
+			 * @return
+			 */
+			IfNull then(Object value);
+
+			/**
+			 * @param fieldReference the field holding the replacement value, must not be {@literal null}.
+			 * @return
+			 */
+			IfNull thenValueOf(String fieldReference);
+
+			/**
+			 * @param expression the expression yielding to the replacement value, must not be {@literal null}.
+			 * @return
+			 */
+			public IfNull thenValueOf(AggregationExpression expression);
+		}
+
+		/**
+		 * Builder for fluent {@link IfNullOperator} creation.
+		 *
+		 * @author Mark Paluch
+		 */
+		static final class IfNullOperatorBuilder implements IfNullBuilder, ThenBuilder {
+
+			private Object condition;
+
+			private IfNullOperatorBuilder() {}
+
+			/**
+			 * Creates a new builder for {@link IfNullOperator}.
+			 *
+			 * @return never {@literal null}.
+			 */
+			public static IfNullOperatorBuilder newBuilder() {
+				return new IfNullOperatorBuilder();
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull.IfNullBuilder#ifNull(java.lang.String)
+			 */
+			public ThenBuilder ifNull(String fieldReference) {
+
+				Assert.hasText(fieldReference, "FieldReference name must not be null or empty!");
+				this.condition = Fields.field(fieldReference);
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull.IfNullBuilder#ifNull(org.springframework.data.mongodb.core.aggregation.AggregationExpression)
+			 */
+			@Override
+			public ThenBuilder ifNull(AggregationExpression expression) {
+
+				Assert.notNull(expression, "AggregationExpression name must not be null or empty!");
+				this.condition = expression;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull.ThenBuilder#then(java.lang.Object)
+			 */
+			public IfNull then(Object value) {
+				return new IfNull(condition, value);
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull.ThenBuilder#thenValueOf(java.lang.String)
+			 */
+			public IfNull thenValueOf(String fieldReference) {
+
+				Assert.notNull(fieldReference, "FieldReference must not be null!");
+				return new IfNull(condition, Fields.field(fieldReference));
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull.ThenBuilder#thenValueOf(org.springframework.data.mongodb.core.aggregation.AggregationExpression)
+			 */
+			public IfNull thenValueOf(AggregationExpression expression) {
+
+				Assert.notNull(expression, "Expression must not be null!");
+				return new IfNull(condition, expression);
+			}
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $switch}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Switch extends AbstractAggregationExpression {
+
+		private Switch(java.util.Map<String, Object> values) {
+			super(values);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$switch";
+		}
+
+		/**
+		 * Creates new {@link Switch}.
+		 *
+		 * @param conditions must not be {@literal null}.
+		 */
+		public static Switch switchCases(CaseOperator... conditions) {
+
+			Assert.notNull(conditions, "Conditions must not be null!");
+			return switchCases(Arrays.asList(conditions));
+		}
+
+		/**
+		 * Creates new {@link Switch}.
+		 *
+		 * @param conditions must not be {@literal null}.
+		 */
+		public static Switch switchCases(List<CaseOperator> conditions) {
+
+			Assert.notNull(conditions, "Conditions must not be null!");
+			return new Switch(Collections.<String, Object> singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
+		}
+
+		public Switch defaultTo(Object value) {
+			return new Switch(append("default", value));
+		}
+
+		/**
+		 * Encapsulates the aggregation framework case document inside a {@code $switch}-operation.
+		 */
+		public static class CaseOperator implements AggregationExpression {
+
+			private final AggregationExpression when;
+			private final Object then;
+
+			private CaseOperator(AggregationExpression when, Object then) {
+
+				this.when = when;
+				this.then = then;
+			}
+
+			public static ThenBuilder when(final AggregationExpression condition) {
+
+				Assert.notNull(condition, "Condition must not be null!");
+
+				return new ThenBuilder() {
+
+					@Override
+					public CaseOperator then(Object value) {
+
+						Assert.notNull(value, "Value must not be null!");
+						return new CaseOperator(condition, value);
+					}
+				};
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+			 */
+			@Override
+			public Document toDocument(AggregationOperationContext context) {
+
+				Document dbo = new Document("case", when.toDocument(context));
+
+				if (then instanceof AggregationExpression) {
+					dbo.put("then", ((AggregationExpression) then).toDocument(context));
+				} else if (then instanceof Field) {
+					dbo.put("then", context.getReference((Field) then).toString());
+				} else {
+					dbo.put("then", then);
+				}
+
+				return dbo;
+			}
+
+			/**
+			 * @author Christoph Strobl
+			 */
+			public interface ThenBuilder {
+
+				/**
+				 * Set the then {@literal value}.
+				 *
+				 * @param value must not be {@literal null}.
+				 * @return
+				 */
+				CaseOperator then(Object value);
+			}
+		}
+	}
+
+	/**
+	 * Encapsulates the aggregation framework {@code $cond} operator. A {@link Cond} allows nested conditions
+	 * {@code if-then[if-then-else]-else} using {@link Field}, {@link CriteriaDefinition}, {@link AggregationExpression}
+	 * or a {@link Document custom} condition. Replacement values can be either {@link Field field references},
+	 * {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be converted to a
+	 * simple MongoDB type.
+	 *
+	 * @see http://docs.mongodb.com/manual/reference/operator/aggregation/cond/
+	 * @author Mark Paluch
+	 * @author Christoph Strobl
+	 */
+	public static class Cond implements AggregationExpression {
+
+		private final Object condition;
+		private final Object thenValue;
+		private final Object otherwiseValue;
+
+		/**
+		 * Creates a new {@link Cond} for a given {@link Field} and {@code then}/{@code otherwise} values.
+		 *
+		 * @param condition must not be {@literal null}.
+		 * @param thenValue must not be {@literal null}.
+		 * @param otherwiseValue must not be {@literal null}.
+		 */
+		private Cond(Field condition, Object thenValue, Object otherwiseValue) {
+			this((Object) condition, thenValue, otherwiseValue);
+		}
+
+		/**
+		 * Creates a new {@link Cond} for a given {@link CriteriaDefinition} and {@code then}/{@code otherwise} values.
+		 *
+		 * @param condition must not be {@literal null}.
+		 * @param thenValue must not be {@literal null}.
+		 * @param otherwiseValue must not be {@literal null}.
+		 */
+		private Cond(CriteriaDefinition condition, Object thenValue, Object otherwiseValue) {
+			this((Object) condition, thenValue, otherwiseValue);
+		}
+
+		private Cond(Object condition, Object thenValue, Object otherwiseValue) {
+
+			Assert.notNull(condition, "Condition must not be null!");
+			Assert.notNull(thenValue, "Then value must not be null!");
+			Assert.notNull(otherwiseValue, "Otherwise value must not be null!");
+
+			assertNotBuilder(condition, "Condition");
+			assertNotBuilder(thenValue, "Then value");
+			assertNotBuilder(otherwiseValue, "Otherwise value");
+
+			this.condition = condition;
+			this.thenValue = thenValue;
+			this.otherwiseValue = otherwiseValue;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+
+			Document condObject = new Document();
+
+			condObject.append("if", resolveCriteria(context, condition));
+			condObject.append("then", resolveValue(context, thenValue));
+			condObject.append("else", resolveValue(context, otherwiseValue));
+
+			return new Document("$cond", condObject);
+		}
+
+		private Object resolveValue(AggregationOperationContext context, Object value) {
+
+			if (value instanceof Document || value instanceof Field) {
+				return resolve(context, value);
+			}
+
+			if (value instanceof AggregationExpression) {
+				return ((AggregationExpression) value).toDocument(context);
+			}
+
+			return context.getMappedObject(new Document("$set", value)).get("$set");
+		}
+
+		private Object resolveCriteria(AggregationOperationContext context, Object value) {
+
+			if (value instanceof Document || value instanceof Field) {
+				return resolve(context, value);
+			}
+
+			if (value instanceof AggregationExpression) {
+				return ((AggregationExpression) value).toDocument(context);
+			}
+
+			if (value instanceof CriteriaDefinition) {
+
+				Document mappedObject = context.getMappedObject(((CriteriaDefinition) value).getCriteriaObject());
+				List<Object> clauses = new ArrayList<Object>();
+
+				clauses.addAll(getClauses(context, mappedObject));
+
+				return clauses.size() == 1 ? clauses.get(0) : clauses;
+			}
+
+			throw new InvalidDataAccessApiUsageException(
+					String.format("Invalid value in condition. Supported: Document, Field references, Criteria, got: %s", value));
+		}
+
+		private List<Object> getClauses(AggregationOperationContext context, Document mappedObject) {
+
+			List<Object> clauses = new ArrayList<Object>();
+
+			for (String key : mappedObject.keySet()) {
+
+				Object predicate = mappedObject.get(key);
+				clauses.addAll(getClauses(context, key, predicate));
+			}
+
+			return clauses;
+		}
+
+		private List<Object> getClauses(AggregationOperationContext context, String key, Object predicate) {
+
+			List<Object> clauses = new ArrayList<Object>();
+
+			if (predicate instanceof List) {
+
+				List<Object> args = new ArrayList<Object>();
+				for (Object clause : (List<?>) predicate) {
+					if (clause instanceof Document) {
+						args.addAll(getClauses(context, (Document) clause));
+					}
+				}
+
+				clauses.add(new Document(key, args));
+
+			} else if (predicate instanceof Document) {
+
+				Document nested = (Document) predicate;
+
+				for (String s : nested.keySet()) {
+
+					if (!isKeyword(s)) {
+						continue;
+					}
+
+					List<Object> args = new ArrayList<Object>();
+					args.add("$" + key);
+					args.add(nested.get(s));
+					clauses.add(new Document(s, args));
+				}
+
+			} else if (!isKeyword(key)) {
+
+				List<Object> args = new ArrayList<Object>();
+				args.add("$" + key);
+				args.add(predicate);
+				clauses.add(new Document("$eq", args));
+			}
+
+			return clauses;
+		}
+
+		/**
+		 * Returns whether the given {@link String} is a MongoDB keyword.
+		 *
+		 * @param candidate
+		 * @return
+		 */
+		private boolean isKeyword(String candidate) {
+			return candidate.startsWith("$");
+		}
+
+		private Object resolve(AggregationOperationContext context, Object value) {
+
+			if (value instanceof Document) {
+				return context.getMappedObject((Document) value);
+			}
+
+			return context.getReference((Field) value).toString();
+		}
+
+		private void assertNotBuilder(Object toCheck, String name) {
+			Assert.isTrue(!ClassUtils.isAssignableValue(ConditionalExpressionBuilder.class, toCheck),
+					String.format("%s must not be of type %s", name, ConditionalExpressionBuilder.class.getSimpleName()));
+		}
+
+		/**
+		 * Get a builder that allows fluent creation of {@link Cond}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		public static WhenBuilder newBuilder() {
+			return ConditionalExpressionBuilder.newBuilder();
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the boolean expression used in {@code if}.
+		 *
+		 * @param booleanExpression must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(Document booleanExpression) {
+			return ConditionalExpressionBuilder.newBuilder().when(booleanExpression);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the {@link AggregationExpression} used in {@code if}.
+		 *
+		 * @param expression expression that yields in a boolean result, must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(AggregationExpression expression) {
+			return ConditionalExpressionBuilder.newBuilder().when(expression);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the field reference used in {@code if}.
+		 *
+		 * @param booleanField name of a field holding a boolean value, must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(String booleanField) {
+			return ConditionalExpressionBuilder.newBuilder().when(booleanField);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the {@link CriteriaDefinition} used in {@code if}.
+		 *
+		 * @param criteria criteria to evaluate, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		public static ThenBuilder when(CriteriaDefinition criteria) {
+			return ConditionalExpressionBuilder.newBuilder().when(criteria);
+		}
+
+		/**
+		 * @author Mark Paluch
+		 */
+		public static interface WhenBuilder {
+
+			/**
+			 * @param booleanExpression expression that yields in a boolean result, must not be {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder when(Document booleanExpression);
+
+			/**
+			 * @param expression expression that yields in a boolean result, must not be {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder when(AggregationExpression expression);
+
+			/**
+			 * @param booleanField name of a field holding a boolean value, must not be {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder when(String booleanField);
+
+			/**
+			 * @param criteria criteria to evaluate, must not be {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder when(CriteriaDefinition criteria);
+		}
+
+		/**
+		 * @author Mark Paluch
+		 */
+		public static interface ThenBuilder {
+
+			/**
+			 * @param value the value to be used if the condition evaluates {@literal true}. Can be a {@link Document}, a
+			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 *          must not be {@literal null}.
+			 * @return the {@link OtherwiseBuilder}
+			 */
+			OtherwiseBuilder then(Object value);
+
+			/**
+			 * @param fieldReference must not be {@literal null}.
+			 * @return the {@link OtherwiseBuilder}
+			 */
+			OtherwiseBuilder thenValueOf(String fieldReference);
+
+			/**
+			 * @param expression must not be {@literal null}.
+			 * @return the {@link OtherwiseBuilder}
+			 */
+			OtherwiseBuilder thenValueOf(AggregationExpression expression);
+		}
+
+		/**
+		 * @author Mark Paluch
+		 */
+		public static interface OtherwiseBuilder {
+
+			/**
+			 * @param value the value to be used if the condition evaluates {@literal false}. Can be a {@link Document}, a
+			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 *          must not be {@literal null}.
+			 * @return the {@link Cond}
+			 */
+			Cond otherwise(Object value);
+
+			/**
+			 * @param fieldReference must not be {@literal null}.
+			 * @return the {@link Cond}
+			 */
+			Cond otherwiseValueOf(String fieldReference);
+
+			/**
+			 * @param expression must not be {@literal null}.
+			 * @return the {@link Cond}
+			 */
+			Cond otherwiseValueOf(AggregationExpression expression);
+		}
+
+		/**
+		 * Builder for fluent {@link Cond} creation.
+		 *
+		 * @author Mark Paluch
+		 */
+		static class ConditionalExpressionBuilder implements WhenBuilder, ThenBuilder, OtherwiseBuilder {
+
+			private Object condition;
+			private Object thenValue;
+
+			private ConditionalExpressionBuilder() {}
+
+			/**
+			 * Creates a new builder for {@link Cond}.
+			 *
+			 * @return never {@literal null}.
+			 */
+			public static ConditionalExpressionBuilder newBuilder() {
+				return new ConditionalExpressionBuilder();
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.WhenBuilder#when(org.bson.Document)
+			 */
+			@Override
+			public ConditionalExpressionBuilder when(Document booleanExpression) {
+
+				Assert.notNull(booleanExpression, "'Boolean expression' must not be null!");
+
+				this.condition = booleanExpression;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.WhenBuilder#when(org.springframework.data.mongodb.core.query.CriteriaDefinition)
+			 */
+			@Override
+			public ThenBuilder when(CriteriaDefinition criteria) {
+
+				Assert.notNull(criteria, "Criteria must not be null!");
+				this.condition = criteria;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.WhenBuilder#when(org.springframework.data.mongodb.core.aggregation.AggregationExpression)
+			 */
+			@Override
+			public ThenBuilder when(AggregationExpression expression) {
+
+				Assert.notNull(expression, "AggregationExpression field must not be null!");
+				this.condition = expression;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.WhenBuilder#when(java.lang.String)
+			 */
+			@Override
+			public ThenBuilder when(String booleanField) {
+
+				Assert.hasText(booleanField, "Boolean field name must not be null or empty!");
+				this.condition = Fields.field(booleanField);
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.ThenBuilder#then(java.lang.Object)
+			 */
+			@Override
+			public OtherwiseBuilder then(Object thenValue) {
+
+				Assert.notNull(thenValue, "Then-value must not be null!");
+				this.thenValue = thenValue;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.ThenBuilder#thenValueOf(java.lang.String)
+			 */
+			@Override
+			public OtherwiseBuilder thenValueOf(String fieldReference) {
+
+				Assert.notNull(fieldReference, "FieldReference must not be null!");
+				this.thenValue = Fields.field(fieldReference);
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.ThenBuilder#thenValueOf(org.springframework.data.mongodb.core.aggregation.AggregationExpression)
+			 */
+			@Override
+			public OtherwiseBuilder thenValueOf(AggregationExpression expression) {
+
+				Assert.notNull(expression, "AggregationExpression must not be null!");
+				this.thenValue = expression;
+				return this;
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.OtherwiseBuilder#otherwise(java.lang.Object)
+			 */
+			@Override
+			public Cond otherwise(Object otherwiseValue) {
+
+				Assert.notNull(otherwiseValue, "Value must not be null!");
+				return new Cond(condition, thenValue, otherwiseValue);
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.OtherwiseBuilder#otherwiseValueOf(java.lang.String)
+			 */
+			@Override
+			public Cond otherwiseValueOf(String fieldReference) {
+
+				Assert.notNull(fieldReference, "FieldReference must not be null!");
+				return new Cond(condition, thenValue, Fields.field(fieldReference));
+			}
+
+			/* (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.OtherwiseBuilder#otherwiseValueOf(org.springframework.data.mongodb.core.aggregation.AggregationExpression)
+			 */
+			@Override
+			public Cond otherwiseValueOf(AggregationExpression expression) {
+
+				Assert.notNull(expression, "AggregationExpression must not be null!");
+				return new Cond(condition, thenValue, expression);
+			}
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DataTypeOperators.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal data type} expressions.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ * @soundtrack Clawfinger - Catch Me
+ */
+public class DataTypeOperators {
+
+	/**
+	 * Return the BSON data type of the given {@literal field}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static Type typeOf(String fieldReference) {
+		return Type.typeOf(fieldReference);
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $type}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Type extends AbstractAggregationExpression {
+
+		private Type(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$type";
+		}
+
+		/**
+		 * Creates new {@link Type}.
+		 *
+		 * @param field must not be {@literal null}.
+		 * @return
+		 */
+		public static Type typeOf(String field) {
+
+			Assert.notNull(field, "Field must not be null!");
+			return new Type(Fields.field(field));
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DateOperators.java

@@ -0,0 +1,838 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.LinkedHashMap;
+
+import org.springframework.data.mongodb.core.aggregation.ArithmeticOperators.ArithmeticOperatorFactory;
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal Date} aggregation operations.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+public class DateOperators {
+
+	/**
+	 * Take the date referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static DateOperatorFactory dateOf(String fieldReference) {
+
+		Assert.notNull(fieldReference, "FieldReference must not be null!");
+		return new DateOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the date resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static DateOperatorFactory dateOf(AggregationExpression expression) {
+
+		Assert.notNull(expression, "Expression must not be null!");
+		return new DateOperatorFactory(expression);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class DateOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link ArithmeticOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public DateOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link ArithmeticOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public DateOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the day of the year for a date as a number between 1 and
+		 * 366.
+		 *
+		 * @return
+		 */
+		public DayOfYear dayOfYear() {
+			return usesFieldRef() ? DayOfYear.dayOfYear(fieldReference) : DayOfYear.dayOfYear(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the day of the month for a date as a number between 1 and
+		 * 31.
+		 *
+		 * @return
+		 */
+		public DayOfMonth dayOfMonth() {
+			return usesFieldRef() ? DayOfMonth.dayOfMonth(fieldReference) : DayOfMonth.dayOfMonth(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the day of the week for a date as a number between 1
+		 * (Sunday) and 7 (Saturday).
+		 *
+		 * @return
+		 */
+		public DayOfWeek dayOfWeek() {
+			return usesFieldRef() ? DayOfWeek.dayOfWeek(fieldReference) : DayOfWeek.dayOfWeek(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the year portion of a date.
+		 *
+		 * @return
+		 */
+		public Year year() {
+			return usesFieldRef() ? Year.yearOf(fieldReference) : Year.yearOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the month of a date as a number between 1 and 12.
+		 *
+		 * @return
+		 */
+		public Month month() {
+			return usesFieldRef() ? Month.monthOf(fieldReference) : Month.monthOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the week of the year for a date as a number between 0 and
+		 * 53.
+		 *
+		 * @return
+		 */
+		public Week week() {
+			return usesFieldRef() ? Week.weekOf(fieldReference) : Week.weekOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the hour portion of a date as a number between 0 and 23.
+		 *
+		 * @return
+		 */
+		public Hour hour() {
+			return usesFieldRef() ? Hour.hourOf(fieldReference) : Hour.hourOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the minute portion of a date as a number between 0 and
+		 * 59.
+		 *
+		 * @return
+		 */
+		public Minute minute() {
+			return usesFieldRef() ? Minute.minuteOf(fieldReference) : Minute.minuteOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the second portion of a date as a number between 0 and
+		 * 59, but can be 60 to account for leap seconds.
+		 *
+		 * @return
+		 */
+		public Second second() {
+			return usesFieldRef() ? Second.secondOf(fieldReference) : Second.secondOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the millisecond portion of a date as an integer between 0
+		 * and 999.
+		 *
+		 * @return
+		 */
+		public Millisecond millisecond() {
+			return usesFieldRef() ? Millisecond.millisecondOf(fieldReference) : Millisecond.millisecondOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that converts a date object to a string according to a user-specified
+		 * {@literal format}.
+		 *
+		 * @param format must not be {@literal null}.
+		 * @return
+		 */
+		public DateToString toString(String format) {
+			return (usesFieldRef() ? DateToString.dateOf(fieldReference) : DateToString.dateOf(expression)).toString(format);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the weekday number in ISO 8601 format, ranging from 1
+		 * (for Monday) to 7 (for Sunday).
+		 *
+		 * @return
+		 */
+		public IsoDayOfWeek isoDayOfWeek() {
+			return usesFieldRef() ? IsoDayOfWeek.isoDayOfWeek(fieldReference) : IsoDayOfWeek.isoDayOfWeek(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the week number in ISO 8601 format, ranging from 1 to 53.
+		 *
+		 * @return
+		 */
+		public IsoWeek isoWeek() {
+			return usesFieldRef() ? IsoWeek.isoWeekOf(fieldReference) : IsoWeek.isoWeekOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the year number in ISO 8601 format.
+		 *
+		 * @return
+		 */
+		public IsoWeekYear isoWeekYear() {
+			return usesFieldRef() ? IsoWeekYear.isoWeekYearOf(fieldReference) : IsoWeekYear.isoWeekYearOf(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return fieldReference != null;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $dayOfYear}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DayOfYear extends AbstractAggregationExpression {
+
+		private DayOfYear(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dayOfYear";
+		}
+
+		/**
+		 * Creates new {@link DayOfYear}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfYear dayOfYear(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new DayOfYear(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link DayOfYear}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfYear dayOfYear(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new DayOfYear(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $dayOfMonth}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DayOfMonth extends AbstractAggregationExpression {
+
+		private DayOfMonth(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dayOfMonth";
+		}
+
+		/**
+		 * Creates new {@link DayOfMonth}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfMonth dayOfMonth(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new DayOfMonth(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link DayOfMonth}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfMonth dayOfMonth(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new DayOfMonth(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $dayOfWeek}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DayOfWeek extends AbstractAggregationExpression {
+
+		private DayOfWeek(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dayOfWeek";
+		}
+
+		/**
+		 * Creates new {@link DayOfWeek}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfWeek dayOfWeek(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new DayOfWeek(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link DayOfWeek}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static DayOfWeek dayOfWeek(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new DayOfWeek(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $year}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Year extends AbstractAggregationExpression {
+
+		private Year(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$year";
+		}
+
+		/**
+		 * Creates new {@link Year}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Year yearOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Year(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Year}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Year yearOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Year(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $month}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Month extends AbstractAggregationExpression {
+
+		private Month(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$month";
+		}
+
+		/**
+		 * Creates new {@link Month}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Month monthOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Month(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Month}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Month monthOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Month(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $week}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Week extends AbstractAggregationExpression {
+
+		private Week(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$week";
+		}
+
+		/**
+		 * Creates new {@link Week}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Week weekOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Week(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Week}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Week weekOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Week(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $hour}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Hour extends AbstractAggregationExpression {
+
+		private Hour(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$hour";
+		}
+
+		/**
+		 * Creates new {@link Hour}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Hour hourOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Hour(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Hour}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Hour hourOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Hour(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $minute}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Minute extends AbstractAggregationExpression {
+
+		private Minute(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$minute";
+		}
+
+		/**
+		 * Creates new {@link Minute}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Minute minuteOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Minute(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Minute}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Minute minuteOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Minute(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $second}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Second extends AbstractAggregationExpression {
+
+		private Second(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$second";
+		}
+
+		/**
+		 * Creates new {@link Second}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Second secondOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Second(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Second}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Second secondOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Second(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $millisecond}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Millisecond extends AbstractAggregationExpression {
+
+		private Millisecond(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$millisecond";
+		}
+
+		/**
+		 * Creates new {@link Millisecond}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static Millisecond millisecondOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new Millisecond(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Millisecond}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static Millisecond millisecondOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new Millisecond(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $dateToString}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DateToString extends AbstractAggregationExpression {
+
+		private DateToString(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dateToString";
+		}
+
+		/**
+		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static FormatBuilder dateOf(final String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new FormatBuilder() {
+
+				@Override
+				public DateToString toString(String format) {
+
+					Assert.notNull(format, "Format must not be null!");
+					return new DateToString(argumentMap(Fields.field(fieldReference), format));
+				}
+			};
+		}
+
+		/**
+		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static FormatBuilder dateOf(final AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new FormatBuilder() {
+
+				@Override
+				public DateToString toString(String format) {
+
+					Assert.notNull(format, "Format must not be null!");
+					return new DateToString(argumentMap(expression, format));
+				}
+			};
+		}
+
+		private static java.util.Map<String, Object> argumentMap(Object date, String format) {
+
+			java.util.Map<String, Object> args = new LinkedHashMap<String, Object>(2);
+			args.put("format", format);
+			args.put("date", date);
+			return args;
+		}
+
+		public interface FormatBuilder {
+
+			/**
+			 * Creates new {@link DateToString} with all previously added arguments appending the given one.
+			 *
+			 * @param format must not be {@literal null}.
+			 * @return
+			 */
+			DateToString toString(String format);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $isoDayOfWeek}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class IsoDayOfWeek extends AbstractAggregationExpression {
+
+		private IsoDayOfWeek(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$isoDayOfWeek";
+		}
+
+		/**
+		 * Creates new {@link IsoDayOfWeek}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoDayOfWeek isoDayOfWeek(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new IsoDayOfWeek(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link IsoDayOfWeek}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoDayOfWeek isoDayOfWeek(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new IsoDayOfWeek(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $isoWeek}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class IsoWeek extends AbstractAggregationExpression {
+
+		private IsoWeek(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$isoWeek";
+		}
+
+		/**
+		 * Creates new {@link IsoWeek}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoWeek isoWeekOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new IsoWeek(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link IsoWeek}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoWeek isoWeekOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new IsoWeek(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $isoWeekYear}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class IsoWeekYear extends AbstractAggregationExpression {
+
+		private IsoWeekYear(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$isoWeekYear";
+		}
+
+		/**
+		 * Creates new {@link IsoWeekYear}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoWeekYear isoWeekYearOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new IsoWeekYear(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link Millisecond}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static IsoWeekYear isoWeekYearOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new IsoWeekYear(expression);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ExposedFieldsAggregationOperationContext.java

@@ -55,7 +55,7 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 
 	/* 
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperationContext#getMappedObject(com.mongodb.Document)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperationContext#getMappedObject(org.bson.Document)
 	 */
 	@Override
 	public Document getMappedObject(Document document) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java

@@ -27,9 +27,6 @@ import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldRefe
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
-import com.mongodb.BasicDBObject;
-import com.mongodb.DBObject;
-
 /**
  * Encapsulates the aggregation framework {@code $group}-operation.
  * <p>

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/LiteralOperators.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal literal} aggregation operations.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+public class LiteralOperators {
+
+	/**
+	 * Take the value referenced by given {@literal value}.
+	 *
+	 * @param value must not be {@literal null}.
+	 * @return
+	 */
+	public static LiteralOperatorFactory valueOf(Object value) {
+
+		Assert.notNull(value, "Value must not be null!");
+		return new LiteralOperatorFactory(value);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class LiteralOperatorFactory {
+
+		private final Object value;
+
+		/**
+		 * Creates new {@link LiteralOperatorFactory} for given {@literal value}.
+		 *
+		 * @param value must not be {@literal null}.
+		 */
+		public LiteralOperatorFactory(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			this.value = value;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that returns the associated value without parsing.
+		 *
+		 * @return
+		 */
+		public Literal asLiteral() {
+			return Literal.asLiteral(value);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $literal}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class Literal extends AbstractAggregationExpression {
+
+		private Literal(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$literal";
+		}
+
+		/**
+		 * Creates new {@link Literal}.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return
+		 */
+		public static Literal asLiteral(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			return new Literal(value);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/NestedDelegatingExpressionAggregationOperationContext.java

@@ -20,8 +20,6 @@ import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldRefe
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExpressionFieldReference;
 import org.springframework.util.Assert;
 
-import com.mongodb.DBObject;
-
 /**
  * {@link AggregationOperationContext} that delegates {@link FieldReference} resolution and mapping to a parent one, but
  * assures {@link FieldReference} get converted into {@link ExpressionFieldReference} using {@code $$} to ref an inner

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java

@@ -22,9 +22,9 @@ import java.util.Collections;
 import java.util.List;
 
 import org.bson.Document;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Let.ExpressionVariable;
+import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.IfNull;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.IfNull;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.Fields.AggregationField;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder.FieldProjection;
@@ -788,7 +788,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder filter(String as, AggregationExpression condition) {
-			return this.operation.and(AggregationExpressions.Filter.filter(name).as(as).by(condition));
+			return this.operation.and(ArrayOperators.Filter.filter(name).as(as).by(condition));
 		}
 
 		// SET OPERATORS
@@ -893,7 +893,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder absoluteValue() {
-			return this.operation.and(AggregationExpressions.Abs.absoluteValueOf(name));
+			return this.operation.and(ArithmeticOperators.Abs.absoluteValueOf(name));
 		}
 
 		/**
@@ -904,7 +904,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder ceil() {
-			return this.operation.and(AggregationExpressions.Ceil.ceilValueOf(name));
+			return this.operation.and(ArithmeticOperators.Ceil.ceilValueOf(name));
 		}
 
 		/**
@@ -915,7 +915,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder exp() {
-			return this.operation.and(AggregationExpressions.Exp.expValueOf(name));
+			return this.operation.and(ArithmeticOperators.Exp.expValueOf(name));
 		}
 
 		/**
@@ -926,7 +926,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder floor() {
-			return this.operation.and(AggregationExpressions.Floor.floorValueOf(name));
+			return this.operation.and(ArithmeticOperators.Floor.floorValueOf(name));
 		}
 
 		/**
@@ -937,7 +937,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder ln() {
-			return this.operation.and(AggregationExpressions.Ln.lnValueOf(name));
+			return this.operation.and(ArithmeticOperators.Ln.lnValueOf(name));
 		}
 
 		/**
@@ -949,7 +949,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder log(String baseFieldRef) {
-			return this.operation.and(AggregationExpressions.Log.valueOf(name).log(baseFieldRef));
+			return this.operation.and(ArithmeticOperators.Log.valueOf(name).log(baseFieldRef));
 		}
 
 		/**
@@ -961,7 +961,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder log(Number base) {
-			return this.operation.and(AggregationExpressions.Log.valueOf(name).log(base));
+			return this.operation.and(ArithmeticOperators.Log.valueOf(name).log(base));
 		}
 
 		/**
@@ -973,7 +973,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder log(AggregationExpression base) {
-			return this.operation.and(AggregationExpressions.Log.valueOf(name).log(base));
+			return this.operation.and(ArithmeticOperators.Log.valueOf(name).log(base));
 		}
 
 		/**
@@ -984,7 +984,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder log10() {
-			return this.operation.and(AggregationExpressions.Log10.log10ValueOf(name));
+			return this.operation.and(ArithmeticOperators.Log10.log10ValueOf(name));
 		}
 
 		/**
@@ -996,7 +996,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder pow(String exponentFieldRef) {
-			return this.operation.and(AggregationExpressions.Pow.valueOf(name).pow(exponentFieldRef));
+			return this.operation.and(ArithmeticOperators.Pow.valueOf(name).pow(exponentFieldRef));
 		}
 
 		/**
@@ -1008,7 +1008,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder pow(Number exponent) {
-			return this.operation.and(AggregationExpressions.Pow.valueOf(name).pow(exponent));
+			return this.operation.and(ArithmeticOperators.Pow.valueOf(name).pow(exponent));
 		}
 
 		/**
@@ -1020,7 +1020,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder pow(AggregationExpression exponentExpression) {
-			return this.operation.and(AggregationExpressions.Pow.valueOf(name).pow(exponentExpression));
+			return this.operation.and(ArithmeticOperators.Pow.valueOf(name).pow(exponentExpression));
 		}
 
 		/**
@@ -1031,7 +1031,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder sqrt() {
-			return this.operation.and(AggregationExpressions.Sqrt.sqrtOf(name));
+			return this.operation.and(ArithmeticOperators.Sqrt.sqrtOf(name));
 		}
 
 		/**
@@ -1041,7 +1041,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder trunc() {
-			return this.operation.and(AggregationExpressions.Trunc.truncValueOf(name));
+			return this.operation.and(ArithmeticOperators.Trunc.truncValueOf(name));
 		}
 
 		/**
@@ -1088,7 +1088,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder toLower() {
-			return this.operation.and(AggregationExpressions.ToLower.lowerValueOf(name));
+			return this.operation.and(StringOperators.ToLower.lowerValueOf(name));
 		}
 
 		/**
@@ -1099,7 +1099,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder toUpper() {
-			return this.operation.and(AggregationExpressions.ToUpper.upperValueOf(name));
+			return this.operation.and(StringOperators.ToUpper.upperValueOf(name));
 		}
 
 		/**
@@ -1170,7 +1170,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder isArray() {
-			return this.operation.and(AggregationExpressions.IsArray.isArray(name));
+			return this.operation.and(ArrayOperators.IsArray.isArray(name));
 		}
 
 		/**
@@ -1180,7 +1180,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder asLiteral() {
-			return this.operation.and(AggregationExpressions.Literal.asLiteral(name));
+			return this.operation.and(LiteralOperators.Literal.asLiteral(name));
 		}
 
 		/**
@@ -1192,7 +1192,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder dateAsFormattedString(String format) {
-			return this.operation.and(AggregationExpressions.DateToString.dateOf(name).toString(format));
+			return this.operation.and(DateOperators.DateToString.dateOf(name).toString(format));
 		}
 
 		/**
@@ -1207,7 +1207,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 */
 		public ProjectionOperationBuilder let(AggregationExpression valueExpression, String variableName,
 											  AggregationExpression in) {
-			return this.operation.and(AggregationExpressions.Let.define(ExpressionVariable.newVariable(variableName).forExpression(valueExpression)).andApply(in));
+			return this.operation.and(VariableOperators.Let.define(ExpressionVariable.newVariable(variableName).forExpression(valueExpression)).andApply(in));
 		}
 
 		/**
@@ -1220,7 +1220,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @since 1.10
 		 */
 		public ProjectionOperationBuilder let(Collection<ExpressionVariable> variables, AggregationExpression in) {
-			return this.operation.and(AggregationExpressions.Let.define(variables).andApply(in));
+			return this.operation.and(VariableOperators.Let.define(variables).andApply(in));
 		}
 
 		/*

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetOperators.java

@@ -0,0 +1,666 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Sum;
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal Set expressions} which perform {@literal set} operation on arrays, treating arrays as sets.
+ *
+ * @author Christoph Strobl
+ * @since 1.10
+ */
+public class SetOperators {
+
+	/**
+	 * Take the array referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static SetOperatorFactory arrayAsSet(String fieldReference) {
+		return new SetOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the array resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static SetOperatorFactory arrayAsSet(AggregationExpression expression) {
+		return new SetOperatorFactory(expression);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class SetOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link SetOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public SetOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link SetOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public SetOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares the previously mentioned field to one or more arrays and
+		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetEquals isEqualTo(String... arrayReferences) {
+			return createSetEquals().isEqualTo(arrayReferences);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that compares the previously mentioned field to one or more arrays and
+		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetEquals isEqualTo(AggregationExpression... expressions) {
+			return createSetEquals().isEqualTo(expressions);
+		}
+
+		private SetEquals createSetEquals() {
+			return usesFieldRef() ? SetEquals.arrayAsSet(fieldReference) : SetEquals.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and one or more
+		 * arrays and returns an array that contains the elements that appear in every of those.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetIntersection intersects(String... arrayReferences) {
+			return createSetIntersection().intersects(arrayReferences);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and one or more
+		 * arrays and returns an array that contains the elements that appear in every of those.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetIntersection intersects(AggregationExpression... expressions) {
+			return createSetIntersection().intersects(expressions);
+		}
+
+		private SetIntersection createSetIntersection() {
+			return usesFieldRef() ? SetIntersection.arrayAsSet(fieldReference) : SetIntersection.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and one or more
+		 * arrays and returns an array that contains the elements that appear in any of those.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetUnion union(String... arrayReferences) {
+			return createSetUnion().union(arrayReferences);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and one or more
+		 * arrays and returns an array that contains the elements that appear in any of those.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetUnion union(AggregationExpression... expressions) {
+			return createSetUnion().union(expressions);
+		}
+
+		private SetUnion createSetUnion() {
+			return usesFieldRef() ? SetUnion.arrayAsSet(fieldReference) : SetUnion.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and returns an
+		 * array containing the elements that do not exist in the given {@literal arrayReference}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public SetDifference differenceTo(String arrayReference) {
+			return createSetDifference().differenceTo(arrayReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and returns an
+		 * array containing the elements that do not exist in the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public SetDifference differenceTo(AggregationExpression expression) {
+			return createSetDifference().differenceTo(expression);
+		}
+
+		private SetDifference createSetDifference() {
+			return usesFieldRef() ? SetDifference.arrayAsSet(fieldReference) : SetDifference.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and returns
+		 * {@literal true} if it is a subset of the given {@literal arrayReference}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public SetIsSubset isSubsetOf(String arrayReference) {
+			return createSetIsSubset().isSubsetOf(arrayReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and returns
+		 * {@literal true} if it is a subset of the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public SetIsSubset isSubsetOf(AggregationExpression expression) {
+			return createSetIsSubset().isSubsetOf(expression);
+		}
+
+		private SetIsSubset createSetIsSubset() {
+			return usesFieldRef() ? SetIsSubset.arrayAsSet(fieldReference) : SetIsSubset.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that takes array of the previously mentioned field and returns
+		 * {@literal true} if any of the elements are {@literal true} and {@literal false} otherwise.
+		 *
+		 * @return
+		 */
+		public AnyElementTrue anyElementTrue() {
+			return usesFieldRef() ? AnyElementTrue.arrayAsSet(fieldReference) : AnyElementTrue.arrayAsSet(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpressions} that tkes array of the previously mentioned field and returns
+		 * {@literal true} if no elements is {@literal false}.
+		 *
+		 * @return
+		 */
+		public AllElementsTrue allElementsTrue() {
+			return usesFieldRef() ? AllElementsTrue.arrayAsSet(fieldReference) : AllElementsTrue.arrayAsSet(expression);
+		}
+
+		private boolean usesFieldRef() {
+			return this.fieldReference != null;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $setEquals}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetEquals extends AbstractAggregationExpression {
+
+		private SetEquals(List<?> arrays) {
+			super(arrays);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$setEquals";
+		}
+
+		/**
+		 * Create new {@link SetEquals}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static SetEquals arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetEquals(asFields(arrayReference));
+		}
+
+		/**
+		 * Create new {@link SetEquals}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static SetEquals arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetEquals(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link java.util.Set} with all previously added arguments appending the given one.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetEquals isEqualTo(String... arrayReferences) {
+
+			Assert.notNull(arrayReferences, "ArrayReferences must not be null!");
+			return new SetEquals(append(Fields.fields(arrayReferences).asList()));
+		}
+
+		/**
+		 * Creates new {@link Sum} with all previously added arguments appending the given one.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetEquals isEqualTo(AggregationExpression... expressions) {
+
+			Assert.notNull(expressions, "Expressions must not be null!");
+			return new SetEquals(append(Arrays.asList(expressions)));
+		}
+
+		/**
+		 * Creates new {@link Sum} with all previously added arguments appending the given one.
+		 *
+		 * @param array must not be {@literal null}.
+		 * @return
+		 */
+		public SetEquals isEqualTo(Object[] array) {
+
+			Assert.notNull(array, "Array must not be null!");
+			return new SetEquals(append(array));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $setIntersection}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetIntersection extends AbstractAggregationExpression {
+
+		private SetIntersection(List<?> arrays) {
+			super(arrays);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$setIntersection";
+		}
+
+		/**
+		 * Creates new {@link SetIntersection}
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static SetIntersection arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetIntersection(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link SetIntersection}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static SetIntersection arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetIntersection(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetIntersection intersects(String... arrayReferences) {
+
+			Assert.notNull(arrayReferences, "ArrayReferences must not be null!");
+			return new SetIntersection(append(asFields(arrayReferences)));
+		}
+
+		/**
+		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetIntersection intersects(AggregationExpression... expressions) {
+
+			Assert.notNull(expressions, "Expressions must not be null!");
+			return new SetIntersection(append(Arrays.asList(expressions)));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $setUnion}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetUnion extends AbstractAggregationExpression {
+
+		private SetUnion(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$setUnion";
+		}
+
+		/**
+		 * Creates new {@link SetUnion}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static SetUnion arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetUnion(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link SetUnion}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static SetUnion arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetUnion(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
+		 *
+		 * @param arrayReferences must not be {@literal null}.
+		 * @return
+		 */
+		public SetUnion union(String... arrayReferences) {
+
+			Assert.notNull(arrayReferences, "ArrayReferences must not be null!");
+			return new SetUnion(append(asFields(arrayReferences)));
+		}
+
+		/**
+		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return
+		 */
+		public SetUnion union(AggregationExpression... expressions) {
+
+			Assert.notNull(expressions, "Expressions must not be null!");
+			return new SetUnion(append(Arrays.asList(expressions)));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $setDifference}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetDifference extends AbstractAggregationExpression {
+
+		private SetDifference(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$setDifference";
+		}
+
+		/**
+		 * Creates new {@link SetDifference}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static SetDifference arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetDifference(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link SetDifference}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static SetDifference arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetDifference(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public SetDifference differenceTo(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetDifference(append(Fields.field(arrayReference)));
+		}
+
+		/**
+		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public SetDifference differenceTo(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetDifference(append(expression));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $setIsSubset}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetIsSubset extends AbstractAggregationExpression {
+
+		private SetIsSubset(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$setIsSubset";
+		}
+
+		/**
+		 * Creates new {@link SetIsSubset}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static SetIsSubset arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetIsSubset(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link SetIsSubset}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static SetIsSubset arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetIsSubset(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public SetIsSubset isSubsetOf(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new SetIsSubset(append(Fields.field(arrayReference)));
+		}
+
+		/**
+		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public SetIsSubset isSubsetOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new SetIsSubset(append(expression));
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $anyElementTrue}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class AnyElementTrue extends AbstractAggregationExpression {
+
+		private AnyElementTrue(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$anyElementTrue";
+		}
+
+		/**
+		 * Creates new {@link AnyElementTrue}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static AnyElementTrue arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new AnyElementTrue(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link AnyElementTrue}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static AnyElementTrue arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new AnyElementTrue(Collections.singletonList(expression));
+		}
+
+		public AnyElementTrue anyElementTrue() {
+			return this;
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $allElementsTrue}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class AllElementsTrue extends AbstractAggregationExpression {
+
+		private AllElementsTrue(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$allElementsTrue";
+		}
+
+		/**
+		 * Creates new {@link AllElementsTrue}.
+		 *
+		 * @param arrayReference must not be {@literal null}.
+		 * @return
+		 */
+		public static AllElementsTrue arrayAsSet(String arrayReference) {
+
+			Assert.notNull(arrayReference, "ArrayReference must not be null!");
+			return new AllElementsTrue(asFields(arrayReference));
+		}
+
+		/**
+		 * Creates new {@link AllElementsTrue}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return
+		 */
+		public static AllElementsTrue arrayAsSet(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			return new AllElementsTrue(Collections.singletonList(expression));
+		}
+
+		public AllElementsTrue allElementsTrue() {
+			return this;
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/TypeBasedAggregationOperationContext.java

@@ -65,7 +65,7 @@ public class TypeBasedAggregationOperationContext implements AggregationOperatio
 
 	/* 
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperationContext#getMappedObject(com.mongodb.Document)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperationContext#getMappedObject(org.bson.Document)
 	 */
 	@Override
 	public Document getMappedObject(Document document) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/VariableOperators.java

@@ -0,0 +1,389 @@
+/*
+ * Copyright 2016. the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal variable} aggregation operations.
+ *
+ * @author Christoph Strobl
+ * @author Mark Paluch
+ * @since 1.10
+ */
+public class VariableOperators {
+
+	/**
+	 * Starts building new {@link Map} that applies an {@link AggregationExpression} to each item of a referenced array
+	 * and returns an array with the applied results.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return
+	 */
+	public static Map.AsBuilder mapItemsOf(String fieldReference) {
+		return Map.itemsOf(fieldReference);
+	}
+
+	/**
+	 * Starts building new {@link Map} that applies an {@link AggregationExpression} to each item of a referenced array
+	 * and returns an array with the applied results.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return
+	 */
+	public static Map.AsBuilder mapItemsOf(AggregationExpression expression) {
+		return Map.itemsOf(expression);
+	}
+
+	/**
+	 * Start creating new {@link Let} that allows definition of {@link ExpressionVariable} that can be used within a
+	 * nested {@link AggregationExpression}.
+	 *
+	 * @param variables must not be {@literal null}.
+	 * @return
+	 */
+	public static Let.LetBuilder define(ExpressionVariable... variables) {
+		return Let.define(variables);
+	}
+
+	/**
+	 * Start creating new {@link Let} that allows definition of {@link ExpressionVariable} that can be used within a
+	 * nested {@link AggregationExpression}.
+	 *
+	 * @param variables must not be {@literal null}.
+	 * @return
+	 */
+	public static Let.LetBuilder define(Collection<ExpressionVariable> variables) {
+		return Let.define(variables);
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $map}.
+	 */
+	public static class Map implements AggregationExpression {
+
+		private Object sourceArray;
+		private String itemVariableName;
+		private AggregationExpression functionToApply;
+
+		private Map(Object sourceArray, String itemVariableName, AggregationExpression functionToApply) {
+
+			Assert.notNull(sourceArray, "SourceArray must not be null!");
+			Assert.notNull(itemVariableName, "ItemVariableName must not be null!");
+			Assert.notNull(functionToApply, "FunctionToApply must not be null!");
+
+			this.sourceArray = sourceArray;
+			this.itemVariableName = itemVariableName;
+			this.functionToApply = functionToApply;
+		}
+
+		/**
+		 * Starts building new {@link Map} that applies an {@link AggregationExpression} to each item of a referenced array
+		 * and returns an array with the applied results.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return
+		 */
+		public static AsBuilder itemsOf(final String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new AsBuilder() {
+
+				@Override
+				public FunctionBuilder as(final String variableName) {
+
+					Assert.notNull(variableName, "VariableName must not be null!");
+
+					return new FunctionBuilder() {
+
+						@Override
+						public Map andApply(final AggregationExpression expression) {
+
+							Assert.notNull(expression, "AggregationExpression must not be null!");
+							return new Map(Fields.field(fieldReference), variableName, expression);
+						}
+					};
+				}
+
+			};
+		};
+
+		/**
+		 * Starts building new {@link Map} that applies an {@link AggregationExpression} to each item of a referenced array
+		 * and returns an array with the applied results.
+		 *
+		 * @param source must not be {@literal null}.
+		 * @return
+		 */
+		public static AsBuilder itemsOf(final AggregationExpression source) {
+
+			Assert.notNull(source, "AggregationExpression must not be null!");
+
+			return new AsBuilder() {
+
+				@Override
+				public FunctionBuilder as(final String variableName) {
+
+					Assert.notNull(variableName, "VariableName must not be null!");
+
+					return new FunctionBuilder() {
+
+						@Override
+						public Map andApply(final AggregationExpression expression) {
+
+							Assert.notNull(expression, "AggregationExpression must not be null!");
+							return new Map(source, variableName, expression);
+						}
+					};
+				}
+			};
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(final AggregationOperationContext context) {
+			return toMap(ExposedFields.synthetic(Fields.fields(itemVariableName)), context);
+		}
+
+		private Document toMap(ExposedFields exposedFields, AggregationOperationContext context) {
+
+			Document map = new Document();
+			InheritingExposedFieldsAggregationOperationContext operationContext = new InheritingExposedFieldsAggregationOperationContext(
+					exposedFields, context);
+
+			Document input;
+			if (sourceArray instanceof Field) {
+				input = new Document("input", context.getReference((Field) sourceArray).toString());
+			} else {
+				input = new Document("input", ((AggregationExpression) sourceArray).toDocument(context));
+			}
+
+			map.putAll(context.getMappedObject(input));
+			map.put("as", itemVariableName);
+			map.put("in",
+					functionToApply.toDocument(new NestedDelegatingExpressionAggregationOperationContext(operationContext)));
+
+			return new Document("$map", map);
+		}
+
+		public interface AsBuilder {
+
+			/**
+			 * Define the {@literal variableName} for addressing items within the array.
+			 *
+			 * @param variableName must not be {@literal null}.
+			 * @return
+			 */
+			FunctionBuilder as(String variableName);
+		}
+
+		public interface FunctionBuilder {
+
+			/**
+			 * Creates new {@link Map} that applies the given {@link AggregationExpression} to each item of the referenced
+			 * array and returns an array with the applied results.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return
+			 */
+			Map andApply(AggregationExpression expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $let} that binds {@link AggregationExpression} to variables for use in the
+	 * specified {@code in} expression, and returns the result of the expression.
+	 *
+	 * @author Christoph Strobl
+	 * @since 1.10
+	 */
+	public static class Let implements AggregationExpression {
+
+		private final List<ExpressionVariable> vars;
+		private final AggregationExpression expression;
+
+		private Let(List<ExpressionVariable> vars, AggregationExpression expression) {
+
+			this.vars = vars;
+			this.expression = expression;
+		}
+
+		/**
+		 * Start creating new {@link Let} by defining the variables for {@code $vars}.
+		 *
+		 * @param variables must not be {@literal null}.
+		 * @return
+		 */
+		public static LetBuilder define(final Collection<ExpressionVariable> variables) {
+
+			Assert.notNull(variables, "Variables must not be null!");
+
+			return new LetBuilder() {
+
+				@Override
+				public Let andApply(final AggregationExpression expression) {
+
+					Assert.notNull(expression, "Expression must not be null!");
+					return new Let(new ArrayList<ExpressionVariable>(variables), expression);
+				}
+			};
+		}
+
+		/**
+		 * Start creating new {@link Let} by defining the variables for {@code $vars}.
+		 *
+		 * @param variables must not be {@literal null}.
+		 * @return
+		 */
+		public static LetBuilder define(final ExpressionVariable... variables) {
+
+			Assert.notNull(variables, "Variables must not be null!");
+
+			return new LetBuilder() {
+
+				@Override
+				public Let andApply(final AggregationExpression expression) {
+
+					Assert.notNull(expression, "Expression must not be null!");
+					return new Let(Arrays.asList(variables), expression);
+				}
+			};
+		}
+
+		public interface LetBuilder {
+
+			/**
+			 * Define the {@link AggregationExpression} to evaluate.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return
+			 */
+			Let andApply(AggregationExpression expression);
+		}
+
+		/* (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(final AggregationOperationContext context) {
+			return toLet(ExposedFields.synthetic(Fields.fields(getVariableNames())), context);
+		}
+
+		private String[] getVariableNames() {
+
+			String[] varNames = new String[this.vars.size()];
+			for (int i = 0; i < this.vars.size(); i++) {
+				varNames[i] = this.vars.get(i).variableName;
+			}
+
+			return varNames;
+		}
+
+		private Document toLet(ExposedFields exposedFields, AggregationOperationContext context) {
+
+			Document letExpression = new Document();
+			Document mappedVars = new Document();
+			InheritingExposedFieldsAggregationOperationContext operationContext = new InheritingExposedFieldsAggregationOperationContext(
+					exposedFields, context);
+
+			for (ExpressionVariable var : this.vars) {
+				mappedVars.putAll(getMappedVariable(var, context));
+			}
+
+			letExpression.put("vars", mappedVars);
+			letExpression.put("in", getMappedIn(operationContext));
+
+			return new Document("$let", letExpression);
+		}
+
+		private Document getMappedVariable(ExpressionVariable var, AggregationOperationContext context) {
+
+			return new Document(var.variableName, var.expression instanceof AggregationExpression
+					? ((AggregationExpression) var.expression).toDocument(context) : var.expression);
+		}
+
+		private Object getMappedIn(AggregationOperationContext context) {
+			return expression.toDocument(new NestedDelegatingExpressionAggregationOperationContext(context));
+		}
+
+		/**
+		 * @author Christoph Strobl
+		 */
+		public static class ExpressionVariable {
+
+			private final String variableName;
+			private final Object expression;
+
+			/**
+			 * Creates new {@link ExpressionVariable}.
+			 *
+			 * @param variableName can be {@literal null}.
+			 * @param expression can be {@literal null}.
+			 */
+			private ExpressionVariable(String variableName, Object expression) {
+
+				this.variableName = variableName;
+				this.expression = expression;
+			}
+
+			/**
+			 * Create a new {@link ExpressionVariable} with given name.
+			 *
+			 * @param variableName must not be {@literal null}.
+			 * @return never {@literal null}.
+			 */
+			public static ExpressionVariable newVariable(String variableName) {
+
+				Assert.notNull(variableName, "VariableName must not be null!");
+				return new ExpressionVariable(variableName, null);
+			}
+
+			/**
+			 * Create a new {@link ExpressionVariable} with current name and given {@literal expression}.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return never {@literal null}.
+			 */
+			public ExpressionVariable forExpression(AggregationExpression expression) {
+
+				Assert.notNull(expression, "Expression must not be null!");
+				return new ExpressionVariable(variableName, expression);
+			}
+
+			/**
+			 * Create a new {@link ExpressionVariable} with current name and given {@literal expressionObject}.
+			 *
+			 * @param expressionObject must not be {@literal null}.
+			 * @return never {@literal null}.
+			 */
+			public ExpressionVariable forExpression(Document expressionObject) {
+
+				Assert.notNull(expressionObject, "Expression must not be null!");
+				return new ExpressionVariable(variableName, expressionObject);
+			}
+		}
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java

@@ -58,11 +58,7 @@ import org.springframework.data.mapping.model.MappingException;
 import org.springframework.data.mongodb.core.CollectionCallback;
 import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.mongodb.core.Venue;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond;
+import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.ConditionalOperators;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Let;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Let.ExpressionVariable;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Multiply;
 import org.springframework.data.mongodb.core.aggregation.AggregationTests.CarDescriptor.Entry;
 import org.springframework.data.mongodb.core.aggregation.BucketAutoOperation.Granularities;
 import org.springframework.data.mongodb.core.index.GeospatialIndex;
@@ -529,7 +525,7 @@ public class AggregationTests {
 		TypedAggregation<InventoryItem> aggregation = newAggregation(InventoryItem.class, //
 				project("item") //
 						.and("discount")//
-						.applyCondition(Cond.newBuilder().when(Criteria.where("qty").gte(250)) //
+						.applyCondition(ConditionalOperators.Cond.newBuilder().when(Criteria.where("qty").gte(250)) //
 								.then(30) //
 								.otherwise(20)));
 
@@ -1616,11 +1612,11 @@ public class AggregationTests {
 		ExpressionVariable total = ExpressionVariable.newVariable("total")
 				.forExpression(AggregationFunctionExpressions.ADD.of(Fields.field("price"), Fields.field("tax")));
 		ExpressionVariable discounted = ExpressionVariable.newVariable("discounted")
-				.forExpression(Cond.when("applyDiscount").then(0.9D).otherwise(1.0D));
+				.forExpression(ConditionalOperators.Cond.when("applyDiscount").then(0.9D).otherwise(1.0D));
 
 		TypedAggregation<Sales2> agg = Aggregation.newAggregation(Sales2.class,
 				Aggregation.project()
-						.and(Let.define(total, discounted).andApply(
+						.and(VariableOperators.Let.define(total, discounted).andApply(
 								AggregationFunctionExpressions.MULTIPLY.of(Fields.field("total"), Fields.field("discounted"))))
 						.as("finalTotal"));
 
@@ -1720,7 +1716,7 @@ public class AggregationTests {
 		mongoTemplate.insert(Arrays.asList(a1, a2, a3, a4), Art.class);
 
 		TypedAggregation<Art> aggregation = newAggregation(Art.class, //
-				bucketAuto(Multiply.valueOf("price").multiplyBy(10), 3) //
+				bucketAuto(ArithmeticOperators.Multiply.valueOf("price").multiplyBy(10), 3) //
 						.withGranularity(Granularities.E12) //
 						.andOutputCount().as("count") //
 						.andOutput("title").push().as("titles") //
@@ -1756,7 +1752,7 @@ public class AggregationTests {
 
 		mongoTemplate.insert(Arrays.asList(a1, a2, a3, a4), Art.class);
 
-		BucketAutoOperation bucketPrice = bucketAuto(Multiply.valueOf("price").multiplyBy(10), 3) //
+		BucketAutoOperation bucketPrice = bucketAuto(ArithmeticOperators.Multiply.valueOf("price").multiplyBy(10), 3) //
 				.withGranularity(Granularities.E12) //
 				.andOutputCount().as("count") //
 				.andOutput("title").push().as("titles") //

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationUnitTests.java

@@ -31,8 +31,7 @@ import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
 import org.springframework.data.domain.Sort.Direction;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.ConditionalOperators;
 import org.springframework.data.mongodb.core.query.Criteria;
 
 /**
@@ -423,7 +422,7 @@ public class AggregationUnitTests {
 	public void shouldRenderProjectionConditionalExpressionCorrectly() {
 
 		Document agg = Aggregation.newAggregation(//
-				project().and(Cond.newBuilder() //
+				project().and(ConditionalOperators.Cond.newBuilder() //
 						.when("isYellow") //
 						.then("bright") //
 						.otherwise("dark")).as("color"))
@@ -446,7 +445,7 @@ public class AggregationUnitTests {
 
 		Document agg = Aggregation.newAggregation(//
 				project().and("color")
-						.applyCondition(Cond.newBuilder() //
+						.applyCondition(ConditionalOperators.Cond.newBuilder() //
 								.when("isYellow") //
 								.then("bright") //
 								.otherwise("dark")))
@@ -470,7 +469,7 @@ public class AggregationUnitTests {
 		Document agg = Aggregation
 				.newAggregation(project()//
 						.and("color")//
-						.applyCondition(Cond.newBuilder().when(Criteria.where("key").gt(5)) //
+						.applyCondition(ConditionalOperators.Cond.newBuilder().when(Criteria.where("key").gt(5)) //
 								.then("bright").otherwise("dark"))) //
 				.toDocument("foo", Aggregation.DEFAULT_CONTEXT);
 
@@ -518,7 +517,7 @@ public class AggregationUnitTests {
 				.newAggregation(//
 						project().and("color").as("chroma"),
 						project().and("luminosity") //
-								.applyCondition(Cond.newBuilder()
+								.applyCondition(ConditionalOperators.Cond.newBuilder()
 										.when(Criteria.where("chroma") //
 												.is(100)) //
 										.then("bright").otherwise("dark"))) //

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/BucketOperationUnitTests.java

@@ -21,7 +21,6 @@ import static org.junit.Assert.*;
 import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
 
 import org.junit.Test;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.ArithmeticOperators;
 
 import org.bson.Document;
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/CondExpressionUnitTests.java

@@ -16,15 +16,14 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import static org.junit.Assert.*;
-import static org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond.*;
+import static org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond.*;
 import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 
 import java.util.Arrays;
 
 import org.bson.Document;
 import org.junit.Test;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Cond;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.ConditionalOperators;
 import org.springframework.data.mongodb.core.query.Criteria;
 
 /**

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/FilterExpressionUnitTests.java

@@ -17,7 +17,7 @@ package org.springframework.data.mongodb.core.aggregation;
 
 import static org.hamcrest.core.Is.*;
 import static org.junit.Assert.*;
-import static org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Filter.*;
+import static org.springframework.data.mongodb.core.aggregation.ArrayOperators.Filter.*;
 
 import java.util.Arrays;
 import java.util.List;

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/GraphLookupOperationUnitTests.java

@@ -22,7 +22,6 @@ import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 import org.bson.Document;
 import org.junit.Test;
 import org.springframework.data.mongodb.core.Person;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Literal;
 import org.springframework.data.mongodb.core.query.Criteria;
 
 import com.mongodb.BasicDBObject;
@@ -112,7 +111,7 @@ public class GraphLookupOperationUnitTests {
 
 		GraphLookupOperation graphLookupOperation = GraphLookupOperation.builder() //
 				.from("employees") //
-				.startWith("reportsTo", Literal.asLiteral("$boss")) //
+				.startWith("reportsTo", LiteralOperators.Literal.asLiteral("$boss")) //
 				.connectFrom("reportsTo") //
 				.connectTo("name") //
 				.as("reportingHierarchy");
@@ -146,7 +145,7 @@ public class GraphLookupOperationUnitTests {
 
 		GraphLookupOperation graphLookupOperation = GraphLookupOperation.builder() //
 				.from("employees") //
-				.startWith(Literal.asLiteral("hello")) //
+				.startWith(LiteralOperators.Literal.asLiteral("hello")) //
 				.connectFrom("reportsTo") //
 				.connectTo("name") //
 				.as("reportingHierarchy");

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperationUnitTests.java

@@ -19,9 +19,9 @@ import static org.hamcrest.Matchers.*;
 import static org.hamcrest.core.Is.is;
 import static org.junit.Assert.*;
 import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
-import static org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Let.ExpressionVariable.*;
 import static org.springframework.data.mongodb.core.aggregation.AggregationFunctionExpressions.*;
 import static org.springframework.data.mongodb.core.aggregation.Fields.*;
+import static org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable.*;
 import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 
 import java.util.Arrays;
@@ -32,12 +32,11 @@ import org.hamcrest.Matchers;
 import org.junit.Test;
 import org.springframework.data.domain.Range;
 import org.springframework.data.mongodb.core.DocumentTestUtils;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Let.ExpressionVariable;
+import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Reduce.PropertyExpression;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Reduce.PropertyExpression;
+import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Reduce.Variable;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Reduce.Variable;
+import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Switch.CaseOperator;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.Switch.CaseOperator;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.*;
+import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
 
 /**
  * Unit tests for {@link ProjectionOperation}.
@@ -1759,7 +1758,8 @@ public class ProjectionOperationUnitTests {
 						.define(
 								newVariable("total")
 										.forExpression(AggregationFunctionExpressions.ADD.of(Fields.field("price"), Fields.field("tax"))),
-								newVariable("discounted").forExpression(Cond.when("applyDiscount").then(0.9D).otherwise(1.0D)))
+								newVariable("discounted")
+										.forExpression(ConditionalOperators.Cond.when("applyDiscount").then(0.9D).otherwise(1.0D)))
 						.andApply(AggregationFunctionExpressions.MULTIPLY.of(Fields.field("total"), Fields.field("discounted")))) //
 				.as("finalTotal").toDocument(Aggregation.DEFAULT_CONTEXT);
 
@@ -1783,7 +1783,7 @@ public class ProjectionOperationUnitTests {
 				.forExpression(AggregationFunctionExpressions.ADD.of(Fields.field("price"), Fields.field("tax")));
 
 		ExpressionVariable var2 = newVariable("discounted")
-				.forExpression(Cond.when("applyDiscount").then(0.9D).otherwise(1.0D));
+				.forExpression(ConditionalOperators.Cond.when("applyDiscount").then(0.9D).otherwise(1.0D));
 
 		Document agg = Aggregation.project().and("foo")
 				.let(Arrays.asList(var1, var2),
@@ -1919,8 +1919,8 @@ public class ProjectionOperationUnitTests {
 	@Test
 	public void shouldRenderRangeCorrectly() {
 
-		Document agg = project().and(RangeOperator.rangeStartingAt(0L).to("distance").withStepSize(25L)).as("rest_stops")
+		Document agg = project().and(ArrayOperators.RangeOperator.rangeStartingAt(0L).to("distance").withStepSize(25L))
-				.toDocument(Aggregation.DEFAULT_CONTEXT);
+				.as("rest_stops").toDocument(Aggregation.DEFAULT_CONTEXT);
 
 		assertThat(agg, isBsonObject().containing("$project.rest_stops.$range.[0]", 0L)
 				.containing("$project.rest_stops.$range.[1]", "$distance").containing("$project.rest_stops.$range.[2]", 25L));
@@ -2068,11 +2068,16 @@ public class ProjectionOperationUnitTests {
 				"   }\n" + //
 				"}";
 
-		CaseOperator cond1 = CaseOperator.when(Gte.valueOf(Avg.avgOf("scores")).greaterThanEqualToValue(90))
+		CaseOperator cond1 = CaseOperator
+				.when(ComparisonOperators.Gte.valueOf(AccumulatorOperators.Avg.avgOf("scores")).greaterThanEqualToValue(90))
 				.then("Doing great!");
-		CaseOperator cond2 = CaseOperator.when(And.and(Gte.valueOf(Avg.avgOf("scores")).greaterThanEqualToValue(80),
+		CaseOperator cond2 = CaseOperator
-				Lt.valueOf(Avg.avgOf("scores")).lessThanValue(90))).then("Doing pretty well.");
+				.when(BooleanOperators.And.and(
-		CaseOperator cond3 = CaseOperator.when(Lt.valueOf(Avg.avgOf("scores")).lessThanValue(80))
+						ComparisonOperators.Gte.valueOf(AccumulatorOperators.Avg.avgOf("scores")).greaterThanEqualToValue(80),
+						ComparisonOperators.Lt.valueOf(AccumulatorOperators.Avg.avgOf("scores")).lessThanValue(90)))
+				.then("Doing pretty well.");
+		CaseOperator cond3 = CaseOperator
+				.when(ComparisonOperators.Lt.valueOf(AccumulatorOperators.Avg.avgOf("scores")).lessThanValue(80))
 				.then("Needs improvement.");
 
 		Document agg = project().and(ConditionalOperators.switchCases(cond1, cond2, cond3).defaultTo("No scores found."))
@@ -2087,7 +2092,7 @@ public class ProjectionOperationUnitTests {
 	@Test
 	public void shouldTypeCorrectly() {
 
-		Document agg = project().and(Type.typeOf("a")).as("a").toDocument(Aggregation.DEFAULT_CONTEXT);
+		Document agg = project().and(DataTypeOperators.Type.typeOf("a")).as("a").toDocument(Aggregation.DEFAULT_CONTEXT);
 
 		assertThat(agg, Matchers.is(Document.parse("{ $project : { a: { $type: \"$a\" } } }")));
 	}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ReplaceRootOperationUnitTests.java

@@ -20,7 +20,6 @@ import static org.junit.Assert.*;
 
 import org.bson.Document;
 import org.junit.Test;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.VariableOperators;
 import org.springframework.data.mongodb.core.aggregation.ReplaceRootOperation.ReplaceRootDocumentOperation;
 
 /**

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/TypeBasedAggregationOperationContextUnitTests.java

@@ -37,7 +37,6 @@ import org.springframework.data.annotation.Id;
 import org.springframework.data.annotation.PersistenceConstructor;
 import org.springframework.data.domain.Sort.Direction;
 import org.springframework.data.mapping.model.MappingException;
-import org.springframework.data.mongodb.core.aggregation.AggregationExpressions.ConditionalOperators;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.DirectFieldReference;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;