DATAMONGO-2377 - Fix handling of $$value and $$this in field exposing aggregation.

Internal field references to $$this and $$value are now no longer mapped against exposed fields which had caused errors before.

Original pull request: #792.

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

@@ -69,6 +69,11 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 	 */
 	@Override
 	public FieldReference getReference(Field field) {
+
+		if(field.isInternal()) {
+			return new DirectFieldReference(new ExposedField(field, true));
+		}
+
 		return getReference(field, field.getTarget());
 	}
 

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

@@ -43,4 +43,12 @@ public interface Field {
 	 * @return
 	 */
 	boolean isAliased();
+
+	/**
+	 * @return true if the field name references a local value such as {@code $$this}.
+	 * @since 2.1.11
+	 */
+	default boolean isInternal() {
+		return false;
+	}
 }

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

@@ -283,6 +283,11 @@ public final class Fields implements Iterable<Field> {
 			return !getName().equals(getTarget());
 		}
 
+		@Override
+		public boolean isInternal() {
+			return getRaw().endsWith("$$this") || getRaw().endsWith("$$value");
+		}
+
 		/**
 		 * @return {@literal true} in case the field name starts with {@code $$}.
 		 * @since 1.10

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

@@ -22,6 +22,7 @@ import static org.springframework.data.mongodb.test.util.Assertions.*;
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 import org.bson.Document;
@@ -569,6 +570,24 @@ public class AggregationUnitTests {
 		assertThat($project.containsKey("plts.ests")).isTrue();
 	}
 
+	@Test // DATAMONGO-2377
+	public void shouldAllowInternal$$thisAnd$$valueReferences() {
+
+		Document untyped = newAggregation( //
+				Arrays.asList( //
+						(group("uid", "data.sourceId") //
+								.push("data.attributeRecords").as("attributeRecordArrays")), //
+						(project() //
+								.and(ArrayOperators.arrayOf("attributeRecordArrays") //
+										.reduce(ArrayOperators.arrayOf("$$value").concat("$$this")) //
+										.startingWith(Collections.emptyList())) //
+								.as("attributeRecordArrays")) //
+				)).toDocument("collection-1", DEFAULT_CONTEXT);
+
+		assertThat(extractPipelineElement(untyped, 1, "$project")).isEqualTo(Document.parse(
+				"{\"attributeRecordArrays\": {\"$reduce\": {\"input\": \"$attributeRecordArrays\", \"initialValue\": [], \"in\": {\"$concatArrays\": [\"$$value\", \"$$this\"]}}}}"));
+	}
+
 	private Document extractPipelineElement(Document agg, int index, String operation) {
 
 		List<Document> pipeline = (List<Document>) agg.get("pipeline");