Add support for `$expr` operator.

Also, allow construction of $match with an AggregationExpression.

Closes #3790

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

@@ -201,4 +201,5 @@ public class AddFieldsOperation extends DocumentEnhancingOperation {
 			AddFieldsOperationBuilder withValueOfExpression(String operation, Object... values);
 		}
 	}
+
 }

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

@@ -498,7 +498,17 @@ public class Aggregation {
 	public static MatchOperation match(CriteriaDefinition criteria) {
 		return new MatchOperation(criteria);
 	}
-
+	
+	/**
+	 * Creates a new {@link MatchOperation}
+	 *
+	 * @return new instance of {@link MatchOperation}.
+	 * @since 1.10
+	 */
+	public static MatchOperation match() {
+		return new MatchOperation();
+	}
+	
 	/**
 	 * Creates a new {@link GeoNearOperation} instance from the given {@link NearQuery} and the {@code distanceField}. The
 	 * {@code distanceField} defines output field that contains the calculated distance.

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

@@ -0,0 +1,109 @@
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.util.Assert;
+
+public class EvaluationOperators {
+	
+	/**
+	 * Take the value resulting from the given fieldReference.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return new instance of {@link EvaluationOperatorFactory}.
+	 */
+	public static EvaluationOperatorFactory valueOf(String fieldReference) {
+		return new EvaluationOperatorFactory(fieldReference);
+	}
+	
+	/**
+	 * Take the value resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link EvaluationOperatorFactory}.
+	 */
+	public static EvaluationOperatorFactory valueOf(AggregationExpression expression) {
+		return new EvaluationOperatorFactory(expression);
+	}
+	
+	public static class EvaluationOperatorFactory {
+		
+		private final String fieldReference;
+		private final AggregationExpression expression;
+		
+		/**
+		 * Creates new {@link EvaluationOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public EvaluationOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		
+		/**
+		 * Creates new {@link EvaluationOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public EvaluationOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+		
+		/**
+		 * Creates new {@link AggregationExpression} that is a valid aggregation expression.
+		 *
+		 * @return new instance of {@link Expr}.
+		 */
+		public Expr expr() {
+			return usesFieldRef() ? Expr.valueOf(fieldReference) : Expr.valueOf(expression);
+		}
+		
+		
+		public static class Expr extends AbstractAggregationExpression {
+
+			private Expr(Object value) {
+				super(value);
+			}
+
+			@Override
+			protected String getMongoMethod() {
+				return "$expr";
+			}
+
+			/**
+			 * Creates new {@link Expr}.
+			 *
+			 * @param fieldReference must not be {@literal null}.
+			 * @return new instance of {@link Expr}.
+			 */
+			public static Expr valueOf(String fieldReference) {
+
+				Assert.notNull(fieldReference, "FieldReference must not be null!");
+				return new Expr(Fields.field(fieldReference));
+			}
+
+			/**
+			 * Creates new {@link Expr}.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return new instance of {@link Expr}.
+			 */
+			public static Expr valueOf(AggregationExpression expression) {
+
+				Assert.notNull(expression, "Expression must not be null!");
+				return new Expr(expression);
+			}
+
+		}
+		
+		private boolean usesFieldRef() {
+			return fieldReference != null;
+		}
+	}
+
+}

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

@@ -16,6 +16,7 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import org.bson.Document;
+import org.springframework.data.mongodb.core.aggregation.EvaluationOperators.EvaluationOperatorFactory.Expr;
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
 import org.springframework.util.Assert;
 
@@ -36,7 +37,16 @@ import org.springframework.util.Assert;
 public class MatchOperation implements AggregationOperation {
 
 	private final CriteriaDefinition criteriaDefinition;
-
+	private final AggregationExpression expression;
+	
+	/**
+	 * Creates a new {@link MatchOperation}
+	 */
+	public MatchOperation() {
+		this.criteriaDefinition = null;
+		this.expression = null;
+	}
+	
 	/**
 	 * Creates a new {@link MatchOperation} for the given {@link CriteriaDefinition}.
 	 *
@@ -46,14 +56,39 @@ public class MatchOperation implements AggregationOperation {
 
 		Assert.notNull(criteriaDefinition, "Criteria must not be null!");
 		this.criteriaDefinition = criteriaDefinition;
+		this.expression = null;
 	}
-
+	
+	/**
+	 * Creates a new {@link MatchOperation} for the given {@link Expression}.
+	 *
+	 * @param criteriaDefinition must not be {@literal null}.
+	 */
+	private MatchOperation(Expr expression) {
+		Assert.notNull(expression, "Expression must not be null!");
+		this.criteriaDefinition = null;
+		this.expression = expression;
+	}
+	
+	/**
+	 * Creates a new {@link MatchOperation} for the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 */
+	public MatchOperation withValueOf(AggregationExpression expression) {
+		Assert.notNull(expression, "Expression must not be null!");
+		return new MatchOperation(EvaluationOperators.valueOf(expression).expr());
+	}
+	
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
+		if(expression != null) {
+			return new Document(getOperator(), expression.toDocument());
+		}
 		return new Document(getOperator(), context.getMappedObject(criteriaDefinition.getCriteriaObject()));
 	}
 

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

@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.bson.Document;
+
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.expression.spel.ast.Projection;
 import org.springframework.util.Assert;

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

@@ -193,5 +193,6 @@ public class SetOperation extends DocumentEnhancingOperation {
 			 */
 			SetOperation withValueOfExpression(String operation, Object... values);
 		}
+
 	}
 }

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

@@ -0,0 +1,26 @@
+package org.springframework.data.mongodb.core.aggregation;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+
+class MatchOperationUnitTests {
+	
+	@Test // DATAMONGO - 3729
+	public void shouldRenderStdDevPopCorrectly() {
+		MatchOperation operation = Aggregation.match().withValueOf(ArithmeticOperators.valueOf("quiz").stdDevPop());
+		assertThat(operation.toDocument(Aggregation.DEFAULT_CONTEXT)).
+			isEqualTo(Document.parse("{ $match: { \"$expr\" : { \"$stdDevPop\" : \"$quiz\" } } } "));
+		
+	}
+	
+	@Test // DATAMONGO - 3729
+	public void shouldRenderStdDevSampCorrectly() {
+		MatchOperation operation = Aggregation.match().withValueOf(ArithmeticOperators.valueOf("quiz").stdDevSamp());
+		assertThat(operation.toDocument(Aggregation.DEFAULT_CONTEXT)).
+			isEqualTo(Document.parse("{ $match: { \"$expr\" : { \"$stdDevSamp\" : \"$quiz\" } } } "));
+		
+	}
+
+}

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

@@ -21,6 +21,7 @@ import java.util.List;
 
 import org.bson.Document;
 import org.junit.jupiter.api.Test;
+
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
 import org.springframework.data.mongodb.core.convert.QueryMapper;