DATAMONGO-2644 - ProjectOperation no longer errors on inclusion of default _id field.

Original pull request: #890.

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

@@ -1450,6 +1450,14 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 						return field.getTarget();
 					}
 
+					if (field.getTarget().equals(Fields.UNDERSCORE_ID)) {
+						try {
+							return context.getReference(field).getReferenceValue();
+						} catch (java.lang.IllegalArgumentException e) {
+							return Fields.UNDERSCORE_ID_REF;
+						}
+					}
+
 					// check whether referenced field exists in the context
 					return context.getReference(field).getReferenceValue();
 

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

@@ -29,6 +29,11 @@ import org.bson.Document;
 import org.junit.jupiter.api.Test;
 import org.springframework.data.domain.Sort.Direction;
 import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond;
+import org.springframework.data.mongodb.core.aggregation.ProjectionOperationUnitTests.BookWithFieldAnnotation;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.convert.QueryMapper;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.data.mongodb.core.query.Criteria;
 
 /**
@@ -582,6 +587,17 @@ public class AggregationUnitTests {
 				"{\"attributeRecordArrays\": {\"$reduce\": {\"input\": \"$attributeRecordArrays\", \"initialValue\": [], \"in\": {\"$concatArrays\": [\"$$value\", \"$$this\"]}}}}"));
 	}
 
+	@Test // DATAMONGO-2644
+	void projectOnIdIsAlwaysValid() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		Document target = new Aggregation(bucket("start"), project("_id")).toDocument("collection-1",
+				new RelaxedTypeBasedAggregationOperationContext(BookWithFieldAnnotation.class, mappingContext,
+						new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext))));
+
+		assertThat(extractPipelineElement(target, 1, "$project")).isEqualTo(Document.parse(" { \"_id\" : \"$_id\" }"));
+	}
+
 	private Document extractPipelineElement(Document agg, int index, String operation) {
 
 		List<Document> pipeline = (List<Document>) agg.get("pipeline");