Add support for `$rank` and `$denseRank` aggregation operators.

Closes: #3715
Original pull request: #3741.

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

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2021 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
+ *
+ *      https://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.bson.Document;
+
+/**
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public class DocumentOperators {
+
+	/**
+	 * Obtain the document position (including gaps) relative to others (rank).
+	 *
+	 * @return new instance of {@link Rank}.
+	 * @since 3.3
+	 */
+	public static Rank rank() {
+		return new Rank();
+	}
+
+	/**
+	 * Obtain the document position (without gaps) relative to others (rank).
+	 *
+	 * @return new instance of {@link DenseRank}.
+	 * @since 3.3
+	 */
+	public static DenseRank denseRank() {
+		return new DenseRank();
+	}
+
+	/**
+	 * {@link Rank} resolves the current document position (the rank) relative to other documents. If multiple documents
+	 * occupy the same rank, {@literal $rank} places the document with the subsequent value at a rank with a gap.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class Rank implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$rank", new Document());
+		}
+	}
+
+	/**
+	 * {@link DenseRank} resolves the current document position (the rank) relative to other documents. If multiple
+	 * documents occupy the same rank, {@literal $denseRank} places the document with the subsequent value at the next rank without
+	 * any gaps.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DenseRank implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$denseRank", new Document());
+		}
+	}
+}

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

@@ -500,7 +500,10 @@ class SpelExpressionTransformer implements AggregationExpressionTransformer {
 					dbo.put(methodReference.getArgumentMap()[i++], transform(child, context));
 				}
 				args = dbo;
-			} else {
+			} else if (ObjectUtils.nullSafeEquals(methodReference.getArgumentType(), ArgumentType.EMPTY_DOCUMENT)) {
+				args = new Document();
+			}
+			else {
 
 				List<Object> argList = new ArrayList<Object>();
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -68,6 +68,10 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("lte", arrayArgRef().forOperator("$lte"));
 		map.put("ne", arrayArgRef().forOperator("$ne"));
 
+		// DOCUMENT OPERATORS
+		map.put("rank", emptyRef().forOperator("$rank"));
+		map.put("denseRank", emptyRef().forOperator("$denseRank"));
+
 		// ARITHMETIC OPERATORS
 		map.put("abs", singleArgRef().forOperator("$abs"));
 		map.put("add", arrayArgRef().forOperator("$add"));
@@ -307,6 +311,16 @@ public class MethodReferenceNode extends ExpressionNode {
 			return new AggregationMethodReference(null, ArgumentType.MAP, null);
 		}
 
+		/**
+		 * Create a new {@link AggregationMethodReference} for a {@link ArgumentType#EMPTY_DOCUMENT} argument.
+		 *
+		 * @return never {@literal null}.
+		 * @since 3.3
+		 */
+		static AggregationMethodReference emptyRef() {
+			return new AggregationMethodReference(null, ArgumentType.EMPTY_DOCUMENT, null);
+		}
+
 		/**
 		 * Create a new {@link AggregationMethodReference} for a given {@literal aggregationExpressionOperator} reusing
 		 * previously set arguments.
@@ -342,7 +356,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		 * @since 1.10
 		 */
 		public enum ArgumentType {
-			SINGLE, ARRAY, MAP
+			SINGLE, ARRAY, MAP, EMPTY_DOCUMENT
 		}
 	}
 

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

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2021 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
+ *
+ *      https://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 static org.assertj.core.api.Assertions.*;
+import static org.springframework.data.mongodb.core.aggregation.DocumentOperators.*;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author Christoph Strobl
+ */
+class DocumentOperatorsUnitTests {
+
+	@Test // GH-3715
+	void rendersRank() {
+		assertThat(rank().toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(new Document("$rank", new Document()));
+	}
+
+	@Test // GH-3715
+	void rendersDenseRank() {
+		assertThat(denseRank().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(new Document("$denseRank", new Document()));
+	}
+}

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

@@ -956,6 +956,16 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("covarianceSamp(field1, field2)")).isEqualTo(Document.parse("{ \"$covarianceSamp\" : [\"$field1\", \"$field2\"]}"));
 	}
 
+	@Test // GH-3715
+	void shouldRenderRank() {
+		assertThat(transform("rank()")).isEqualTo(Document.parse("{ $rank : {} }"));
+	}
+
+	@Test // GH-3715
+	void shouldRenderDenseRank() {
+		assertThat(transform("denseRank()")).isEqualTo(Document.parse("{ $denseRank : {} }"));
+	}
+
 	private Object transform(String expression, Object... params) {
 		Object result = transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 		return result == null ? null : (!(result instanceof org.bson.Document) ? result.toString() : result);