Polishing.

Simplify field creation considering simplified projection expressions.

See #3917
Original pull request: #4328

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

@@ -97,22 +97,12 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 		}
 
 		if (rootContext instanceof RelaxedTypeBasedAggregationOperationContext) {
-			return new DirectFieldReference(new ExposedField(new Field() {
-				@Override
-				public String getName() {
-					return name;
-				}
 
-				@Override
-				public String getTarget() {
-					return field != null ? field.getTarget() : name;
+			if (field != null) {
+				return new DirectFieldReference(new ExposedField(field, true));
 			}
 
-				@Override
-				public boolean isAliased() {
-					return true;
-				}
-			}, true));
+			return new DirectFieldReference(new ExposedField(name, true));
 		}
 
 		throw new IllegalArgumentException(String.format("Invalid reference '%s'", name));

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

@@ -601,7 +601,7 @@ public class AggregationUnitTests {
 				new RelaxedTypeBasedAggregationOperationContext(BookWithFieldAnnotation.class, mappingContext,
 						new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext))));
 
-		assertThat(extractPipelineElement(target, 1, "$project")).isEqualTo(Document.parse(" { \"_id\" : \"$_id\" }"));
+		assertThat(extractPipelineElement(target, 1, "$project")).isEqualTo(Document.parse(" { \"_id\" : 1 }"));
 	}
 
 	@Test // GH-3898