Support aggregation operators $first and $last via expression method reference.

This commit registers the first(...) and last(...) methods for transformation via SpEL.
Also update reference and java documentation and add issue reference to tests.

Original Pull Request: #3866

Author: christophstrobl

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

@@ -35,6 +35,7 @@
  * @author Christoph Strobl
  * @author Mark Paluch
  * @author Shashank Sharma
+ * @author Divya Srivastava
  * @since 1.0
  */
 public class ArrayOperators {
@@ -364,10 +365,11 @@ public ArrayToObject toObject() {
 		}
 
 		/**
-		 * Creates new {@link AggregationExpression} that return the first element in the given array.
+		 * Creates new {@link AggregationExpression} that return the first element in the associated array.
 		 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
 		 *
 		 * @return new instance of {@link First}.
+		 * @since 3.4
 		 */
 		public First first() {
 
@@ -382,7 +384,8 @@ public First first() {
 		 * Creates new {@link AggregationExpression} that return the last element in the given array.
 		 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
 		 *
-		 * @return new instance of {@link First}.
+		 * @return new instance of {@link Last}.
+		 * @since 3.4
 		 */
 		public Last last() {
 
@@ -1847,6 +1850,9 @@ protected String getMongoMethod() {
 	 * {@link AggregationExpression} for {@code $first} that returns the first element in an array. <br />
 	 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
 	 *
+	 * @author Divya Srivastava
+	 * @author Christoph Strobl
+	 * @since 3.4
 	 */
 	public static class First extends AbstractAggregationExpression {
 
@@ -1875,7 +1881,7 @@ public static First firstOf(String fieldReference) {
 		}
 
 		/**
-		 * Returns the first element of the array of the given {@link AggregationExpression expression}.
+		 * Returns the first element of the array computed by the given {@link AggregationExpression expression}.
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @return new instance of {@link First}.
@@ -1898,6 +1904,9 @@ protected String getMongoMethod() {
 	 * {@link AggregationExpression} for {@code $last} that returns the last element in an array. <br />
 	 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
 	 *
+	 * @author Divya Srivastava
+	 * @author Christoph Strobl
+	 * @since 3.4
 	 */
 	public static class Last extends AbstractAggregationExpression {
 
@@ -1906,7 +1915,7 @@ private Last(Object value) {
 		}
 
 		/**
-		 * Returns the first element in the given array.
+		 * Returns the last element in the given array.
 		 *
 		 * @param array must not be {@literal null}.
 		 * @return new instance of {@link Last}.
@@ -1926,7 +1935,7 @@ public static Last lastOf(String fieldReference) {
 		}
 
 		/**
-		 * Returns the last element of the array of the given {@link AggregationExpression expression}.
+		 * Returns the last element of the array computed buy the given {@link AggregationExpression expression}.
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @return new instance of {@link Last}.
@@ -1944,5 +1953,4 @@ protected String getMongoMethod() {
 			return "$last";
 		}
 	}
-
 }

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

@@ -137,7 +137,9 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("concatArrays", arrayArgRef().forOperator("$concatArrays"));
 		map.put("filter", mapArgRef().forOperator("$filter") //
 				.mappingParametersTo("input", "as", "cond"));
+		map.put("first", singleArgRef().forOperator("$first"));
 		map.put("isArray", singleArgRef().forOperator("$isArray"));
+		map.put("last", singleArgRef().forOperator("$last"));
 		map.put("size", singleArgRef().forOperator("$size"));
 		map.put("slice", arrayArgRef().forOperator("$slice"));
 		map.put("reverseArray", singleArgRef().forOperator("$reverseArray"));

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

@@ -23,14 +23,14 @@
 
 import org.bson.Document;
 import org.junit.jupiter.api.Test;
-
 import org.springframework.data.mongodb.core.aggregation.ArrayOperators.ArrayToObject;
 
 /**
  * Unit tests for {@link ArrayOperators}
  *
  * @author Christoph Strobl
  * @author Shashank Sharma
+ * @author Divya Srivastava
  * @currentRead Royal Assassin - Robin Hobb
  */
 public class ArrayOperatorsUnitTests {
@@ -130,46 +130,45 @@ public void inWithValueList() {
 				.isEqualTo("{ \"$in\" : [\"$userName\", " + VALUE_LIST_STRING + "] }");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void firstWithValueList() {
 
 		assertThat(ArrayOperators.arrayOf(VALUE_LIST).first().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ \"$first\" : " + VALUE_LIST_STRING + "}");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void firstWithExpression() {
 
 		assertThat(ArrayOperators.arrayOf(EXPRESSION).first().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ \"$first\" : " + EXPRESSION_STRING + "}");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void firstWithFieldReference() {
 
 		assertThat(ArrayOperators.arrayOf("field").first().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ $first : \"$field\" }");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void lastWithValueList() {
 
 		assertThat(ArrayOperators.arrayOf(VALUE_LIST).last().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ \"$last\" : " + VALUE_LIST_STRING + "}");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void lastWithExpression() {
 
 		assertThat(ArrayOperators.arrayOf(EXPRESSION).last().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ \"$last\" : " + EXPRESSION_STRING + "}");
 	}
 
-	@Test
+	@Test // GH-3694
 	public void lastWithFieldReference() {
 
 		assertThat(ArrayOperators.arrayOf("field").last().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ $last : \"$field\" }");
 	}
-
 }

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

@@ -411,6 +411,16 @@ void shouldRenderMethodReferenceNodeArrayElemAt() {
 		assertThat(transform("arrayElemAt(a, 10)")).isEqualTo("{ \"$arrayElemAt\" : [ \"$a\" , 10]}");
 	}
 
+	@Test // GH-3694
+	void shouldRenderMethodReferenceNodeFirst() {
+		assertThat(transform("first(a)")).isEqualTo("{ \"$first\" : \"$a\" }");
+	}
+
+	@Test // GH-3694
+	void shouldRenderMethodReferenceNodeLast() {
+		assertThat(transform("last(a)")).isEqualTo("{ \"$last\" : \"$a\" }");
+	}
+
 	@Test // DATAMONGO-1530
 	void shouldRenderMethodReferenceNodeConcatArrays() {
 		assertThat(transform("concatArrays(a, b, c)"))
@@ -879,7 +889,6 @@ void shouldRenderReplaceAll() {
 				.isEqualTo("{ \"$replaceAll\" : {\"input\" : \"$field\" , \"find\" : \"bar\" , \"replacement\" : \"baz\"}}");
 	}
 
-
 	@Test // DATAMONGO-2077
 	void shouldRenderConvertWithoutOptionalParameters() {
 

src/main/asciidoc/reference/aggregation-framework.adoc

@@ -94,7 +94,7 @@ At the time of this writing, we provide support for the following Aggregation Op
 | `eq` (+++*+++ via `is`), `gt`, `gte`, `lt`, `lte`, `ne`
 
 | Array Aggregation Operators
-| `arrayElementAt`, `arrayToObject`, `concatArrays`, `filter`, `in`, `indexOfArray`, `isArray`, `range`, `reverseArray`, `reduce`, `size`, `slice`, `zip`
+| `arrayElementAt`, `arrayToObject`, `concatArrays`, `filter`, `first`, `in`, `indexOfArray`, `isArray`, `last`, range`, `reverseArray`, `reduce`, `size`, `slice`, `zip`
 
 | Literal Operators
 | `literal`