Add support for $documentNumber aggregation operator.

Closes: #3717
Original pull request: #3741.

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

@@ -18,6 +18,8 @@ package org.springframework.data.mongodb.core.aggregation;
 import org.bson.Document;
 
 /**
+ * Gateway to {@literal document expressions} such as {@literal $rank, $documentNumber, etc.}
+ *
  * @author Christoph Strobl
  * @since 3.3
  */
@@ -43,6 +45,16 @@ public class DocumentOperators {
 		return new DenseRank();
 	}
 
+	/**
+	 * Obtain the current document position.
+	 *
+	 * @return new instance of {@link DocumentNumber}.
+	 * @since 3.3
+	 */
+	public static DocumentNumber documentNumber() {
+		return new DocumentNumber();
+	}
+
 	/**
 	 * {@link Rank} resolves the current document position (the rank) relative to other documents. If multiple documents
 	 * occupy the same rank, {@literal $rank} places the document with the subsequent value at a rank with a gap.
@@ -73,4 +85,18 @@ public class DocumentOperators {
 			return new Document("$denseRank", new Document());
 		}
 	}
+
+	/**
+	 * {@link DocumentNumber} resolves the current document position.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DocumentNumber implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$documentNumber", new Document());
+		}
+	}
 }

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

@@ -71,6 +71,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		// DOCUMENT OPERATORS
 		map.put("rank", emptyRef().forOperator("$rank"));
 		map.put("denseRank", emptyRef().forOperator("$denseRank"));
+		map.put("documentNumber", emptyRef().forOperator("$documentNumber"));
 
 		// ARITHMETIC OPERATORS
 		map.put("abs", singleArgRef().forOperator("$abs"));

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

@@ -36,4 +36,9 @@ class DocumentOperatorsUnitTests {
 		assertThat(denseRank().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo(new Document("$denseRank", new Document()));
 	}
+
+	@Test // GH-3717
+	void rendersDocumentNumber() {
+		assertThat(documentNumber().toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(new Document("$documentNumber", new Document()));
+	}
 }

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

@@ -966,6 +966,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("denseRank()")).isEqualTo(Document.parse("{ $denseRank : {} }"));
 	}
 
+	@Test // GH-3717
+	void shouldRenderDocumentNumber() {
+		assertThat(transform("documentNumber()")).isEqualTo(Document.parse("{ $documentNumber : {} }"));
+	}
+
 	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);