Fix mapping custom field names in downstream stages in TypedAggregation pipelines.

Use the root AggregationOperationContext in nested ExposedFieldsAggregationOperationContext to properly apply mapping for domain properties that use @Field.

Closes #4443
Original Pull Request: #4459

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

@@ -67,6 +67,11 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperationContext#getReference(org.springframework.data.mongodb.core.aggregation.ExposedFields.AvailableField)
 	 */
+	@Override
+	public Document getMappedObject(Document document) {
+		return rootContext.getMappedObject(document);
+	}
+
 	@Override
 	public FieldReference getReference(Field field) {
 

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

@@ -87,6 +87,7 @@ import com.mongodb.client.MongoCollection;
  * @author Maninder Singh
  * @author Sergey Shcherbakov
  * @author Minsu Kim
+ * @author Julia Lee
  */
 @ExtendWith(MongoTemplateExtension.class)
 public class AggregationTests {
@@ -116,7 +117,7 @@ public class AggregationTests {
 
 		mongoTemplate.flush(Product.class, UserWithLikes.class, DATAMONGO753.class, Data.class, DATAMONGO788.class,
 				User.class, Person.class, Reservation.class, Venue.class, MeterData.class, LineItem.class, InventoryItem.class,
-				Sales.class, Sales2.class, Employee.class, Art.class, Venue.class);
+				Sales.class, Sales2.class, Employee.class, Art.class, Venue.class, Item.class);
 
 		mongoTemplate.dropCollection(INPUT_COLLECTION);
 		mongoTemplate.dropCollection("personQueryTemp");
@@ -1962,6 +1963,23 @@ public class AggregationTests {
 		assertThat(aggregate.getMappedResults()).contains(widget);
 	}
 
+	@Test // GH-4443
+	void shouldHonorFieldAliasesForFieldReferencesUsingFieldExposingOperation() {
+
+		Item item1 = Item.builder().itemId("1").tags(Arrays.asList("a", "b")).build();
+		Item item2 = Item.builder().itemId("1").tags(Arrays.asList("a", "c")).build();
+		mongoTemplate.insert(Arrays.asList(item1, item2), Item.class);
+
+		TypedAggregation<Item> aggregation = newAggregation(Item.class,
+				match(where("itemId").is("1")),
+				unwind("tags"),
+				match(where("itemId").is("1").and("tags").is("c")));
+		AggregationResults<Document> results = mongoTemplate.aggregate(aggregation, Document.class);
+		List<Document> mappedResults = results.getMappedResults();
+		assertThat(mappedResults).hasSize(1);
+		assertThat(mappedResults.get(0)).containsEntry("item_id", "1");
+	}
+
 	private void createUsersWithReferencedPersons() {
 
 		mongoTemplate.dropCollection(User.class);
@@ -2220,7 +2238,7 @@ public class AggregationTests {
 		List<Item> items;
 	}
 
-	// DATAMONGO-1491
+	// DATAMONGO-1491, GH-4443
 	@lombok.Data
 	@Builder
 	static class Item {
@@ -2229,6 +2247,7 @@ public class AggregationTests {
 		String itemId;
 		Integer quantity;
 		Long price;
+		List<String> tags = new ArrayList<>();
 	}
 
 	// DATAMONGO-1538

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

@@ -44,6 +44,7 @@ import org.springframework.data.mongodb.core.query.Criteria;
  * @author Thomas Darimont
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Julia Lee
  */
 public class AggregationUnitTests {
 
@@ -607,7 +608,23 @@ public class AggregationUnitTests {
 		RelaxedTypeBasedAggregationOperationContext context = new RelaxedTypeBasedAggregationOperationContext(WithRetypedIdField.class, mappingContext,
 				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
 		Document document = project(WithRetypedIdField.class).toDocument(context);
-		assertThat(document).isEqualTo(new Document("$project", new Document("_id", 1).append("renamed-field", 1)));
+		assertThat(document).isEqualTo(new Document("$project", new Document("_id", 1).append("renamed-field", 1).append("entries", 1)));
+	}
+
+	@Test // GH-4443
+	void fieldsExposingContextShouldUseCustomFieldNameFromRelaxedRootContext() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		RelaxedTypeBasedAggregationOperationContext context = new RelaxedTypeBasedAggregationOperationContext(
+				WithRetypedIdField.class, mappingContext,
+				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
+
+		TypedAggregation<WithRetypedIdField> agg = newAggregation(WithRetypedIdField.class,
+				unwind("entries"), match(where("foo").is("value 2")));
+		List<Document> pipeline = agg.toPipeline(context);
+
+		Document fields = getAsDocument(pipeline.get(1), "$match");
+		assertThat(fields.get("renamed-field")).isEqualTo("value 2");
 	}
 
 	private Document extractPipelineElement(Document agg, int index, String operation) {
@@ -625,5 +642,7 @@ public class AggregationUnitTests {
 		@org.springframework.data.mongodb.core.mapping.Field("renamed-field")
 		private String foo;
 
+		private List<String> entries = new ArrayList<>();
+
 	}
 }