Add support for $firstN aggregation operator.

Closes #4139
Original pull request: #4182.

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

@@ -122,4 +122,98 @@ public class SelectionOperators {
 			return new Bottom(append("output", Arrays.asList(out)));
 		}
 	}
+
+	/**
+	 * {@link AbstractAggregationExpression} to return the {@literal $firstN} elements.
+	 */
+	public static class First extends AbstractAggregationExpression {
+
+		protected First(Object value) {
+			super(value);
+		}
+
+		/**
+		 * @return new instance of {@link First}.
+		 */
+		public static First first() {
+			return new First(Collections.emptyMap()).limit(1);
+		}
+
+		/**
+		 * @return new instance of {@link First}.
+		 */
+		public static First first(int numberOfResults) {
+			return new First(Collections.emptyMap()).limit(numberOfResults);
+		}
+
+		/**
+		 * Limits the number of returned elements to the given value.
+		 *
+		 * @param numberOfResults
+		 * @return new instance of {@link Bottom}.
+		 */
+		public First 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 First limit(AggregationExpression expression) {
+			return limit((Object) expression);
+		}
+
+		private First limit(Object value) {
+			return new First(append("n", value));
+		}
+
+		/**
+		 * Define the field to serve as source.
+		 *
+		 * @param fieldName must not be {@literal null}.
+		 * @return new instance of {@link Bottom}.
+		 */
+		public First 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 First 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 First input(String fieldName) {
+			return new First(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 First input(AggregationExpression expression) {
+			return new First(append("input", expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$firstN";
+		}
+	}
 }

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

@@ -216,6 +216,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				.mappingParametersTo("output", "sortBy"));
 		map.put("bottomN", mapArgRef().forOperator("$bottomN") //
 				.mappingParametersTo("n", "output", "sortBy"));
+		map.put("firstN", mapArgRef().forOperator("$firstN") //
+				.mappingParametersTo("n", "input"));
 
 		// CONVERT OPERATORS
 		map.put("convert", mapArgRef().forOperator("$convert") //
@@ -230,7 +232,6 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("toString", singleArgRef().forOperator("$toString"));
 		map.put("degreesToRadians", singleArgRef().forOperator("$degreesToRadians"));
 
-
 		FUNCTIONS = Collections.unmodifiableMap(map);
 	}
 

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

@@ -89,6 +89,27 @@ class SelectionOperatorUnitTests {
 				"""));
 	}
 
+	@Test // GH-4139
+	void firstNMapsFieldNamesCorrectly() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		RelaxedTypeBasedAggregationOperationContext aggregationContext = new RelaxedTypeBasedAggregationOperationContext(
+				Player.class, mappingContext,
+				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
+
+		Document document = SelectionOperators.First.first(3).of("score").toDocument(aggregationContext);
+
+		assertThat(document).isEqualTo(Document.parse("""
+				{
+				   $firstN:
+				   {
+				      n: 3,
+				      input: "$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

@@ -1184,6 +1184,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("bottomN(3, new String[]{\"$playerId\", \"$score\" }, { \"score\" : -1 })")).isEqualTo("{ $bottomN : { n : 3, output: [ \"$playerId\", \"$score\" ], sortBy: { \"score\": -1 }}}");
 	}
 
+	@Test // GH-4139
+	void shouldRenderFirstN() {
+		assertThat(transform("firstN(3, \"$score\")")).isEqualTo("{ $firstN : { n : 3, input : \"$score\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}