Add support for $lastN aggregation operator.

Closes #4139
Original pull request: #4182.

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

@@ -216,4 +216,98 @@ public class SelectionOperators {
 			return "$firstN";
 		}
 	}
+
+	/**
+	 * {@link AbstractAggregationExpression} to return the {@literal $lastN} elements.
+	 */
+	public static class Last extends AbstractAggregationExpression {
+
+		protected Last(Object value) {
+			super(value);
+		}
+
+		/**
+		 * @return new instance of {@link Last}.
+		 */
+		public static Last last() {
+			return new Last(Collections.emptyMap()).limit(1);
+		}
+
+		/**
+		 * @return new instance of {@link Last}.
+		 */
+		public static Last last(int numberOfResults) {
+			return new Last(Collections.emptyMap()).limit(numberOfResults);
+		}
+
+		/**
+		 * Limits the number of returned elements to the given value.
+		 *
+		 * @param numberOfResults
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last limit(int numberOfResults) {
+			return limit((Object) numberOfResults);
+		}
+
+		/**
+		 * Limits the number of returned elements to the value defined by the given {@link AggregationExpression
+		 * expression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last limit(AggregationExpression expression) {
+			return limit((Object) expression);
+		}
+
+		private Last limit(Object value) {
+			return new Last(append("n", value));
+		}
+
+		/**
+		 * Define the field to serve as source.
+		 *
+		 * @param fieldName must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last of(String fieldName) {
+			return input(fieldName);
+		}
+
+		/**
+		 * Define the expression building the value to serve as source.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last of(AggregationExpression expression) {
+			return input(expression);
+		}
+
+		/**
+		 * Define the field to serve as source.
+		 *
+		 * @param fieldName must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last input(String fieldName) {
+			return new Last(append("input", Fields.field(fieldName)));
+		}
+
+		/**
+		 * Define the expression building the value to serve as source.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public Last input(AggregationExpression expression) {
+			return new Last(append("input", expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$lastN";
+		}
+	}
 }

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

@@ -218,6 +218,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				.mappingParametersTo("n", "output", "sortBy"));
 		map.put("firstN", mapArgRef().forOperator("$firstN") //
 				.mappingParametersTo("n", "input"));
+		map.put("lastN", mapArgRef().forOperator("$lastN") //
+				.mappingParametersTo("n", "input"));
 
 		// CONVERT OPERATORS
 		map.put("convert", mapArgRef().forOperator("$convert") //

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

@@ -110,6 +110,27 @@ class SelectionOperatorUnitTests {
 				"""));
 	}
 
+	@Test // GH-4139
+	void lastNMapsFieldNamesCorrectly() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		RelaxedTypeBasedAggregationOperationContext aggregationContext = new RelaxedTypeBasedAggregationOperationContext(
+				Player.class, mappingContext,
+				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
+
+		Document document = SelectionOperators.Last.last(3).of("score").toDocument(aggregationContext);
+
+		assertThat(document).isEqualTo(Document.parse("""
+				{
+				   $lastN:
+				   {
+				      n: 3,
+				      input: "$s_cor_e"
+				   }
+				}
+				"""));
+	}
+
 	static class Player {
 
 		@Field("player_id") String playerId;

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

@@ -1189,6 +1189,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("firstN(3, \"$score\")")).isEqualTo("{ $firstN : { n : 3, input : \"$score\" }}");
 	}
 
+	@Test // GH-4139
+	void shouldRenderLastN() {
+		assertThat(transform("lastN(3, \"$score\")")).isEqualTo("{ $lastN : { n : 3, input : \"$score\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}