Fix field projection value conversion.

The field projection conversion should actually only map field names and avoid value conversion. In the MongoId case an inclusion parameter (1) was unintentionally converted into its String representation which causes trouble on Mongo 4.4 servers.

Fixes: #3668
Original pull request: #3678.

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -195,19 +195,7 @@ public Document getMappedSort(Document sortObject, @Nullable MongoPersistentEnti
 			return new Document();
 		}
 
-		sortObject = filterUnwrappedObjects(sortObject, entity);
-
-		Document mappedSort = new Document();
-		for (Map.Entry<String, Object> entry : BsonUtils.asMap(sortObject).entrySet()) {
-
-			Field field = createPropertyField(entity, entry.getKey(), mappingContext);
-			if (field.getProperty() != null && field.getProperty().isUnwrapped()) {
-				continue;
-			}
-
-			mappedSort.put(field.getMappedKey(), entry.getValue());
-		}
-
+		Document mappedSort = mapFieldsToPropertyNames(sortObject, entity);
 		mapMetaAttributes(mappedSort, entity, MetaMapping.WHEN_PRESENT);
 		return mappedSort;
 	}
@@ -225,13 +213,30 @@ public Document getMappedFields(Document fieldsObject, @Nullable MongoPersistent
 
 		Assert.notNull(fieldsObject, "FieldsObject must not be null!");
 
-		fieldsObject = filterUnwrappedObjects(fieldsObject, entity);
-
-		Document mappedFields = getMappedObject(fieldsObject, entity);
+		Document mappedFields = mapFieldsToPropertyNames(fieldsObject, entity);
 		mapMetaAttributes(mappedFields, entity, MetaMapping.FORCE);
 		return mappedFields;
 	}
 
+	private Document mapFieldsToPropertyNames(Document fields, @Nullable MongoPersistentEntity<?> entity) {
+
+		if (fields.isEmpty()) {
+			return new Document();
+
+		}
+		Document target = new Document();
+		for (Map.Entry<String, Object> entry : BsonUtils.asMap(filterUnwrappedObjects(fields, entity)).entrySet()) {
+
+			Field field = createPropertyField(entity, entry.getKey(), mappingContext);
+			if (field.getProperty() != null && field.getProperty().isUnwrapped()) {
+				continue;
+			}
+
+			target.put(field.getMappedKey(), entry.getValue());
+		}
+		return target;
+	}
+
 	private void mapMetaAttributes(Document source, @Nullable MongoPersistentEntity<?> entity, MetaMapping metaMapping) {
 
 		if (entity == null) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -50,6 +50,7 @@
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.mapping.FieldType;
+import org.springframework.data.mongodb.core.mapping.MongoId;
 import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.TextScore;
@@ -1304,6 +1305,13 @@ void allowsUsingFieldPathsForPropertiesHavingCustomConversionRegistered() {
 		assertThat(mapper.getMappedSort(query.getQueryObject(), context.getPersistentEntity(Customer.class))).isEqualTo(new org.bson.Document("address.street", "1007 Mountain Drive"));
 	}
 
+	@Test // GH-3668
+	void mapStringIdFieldProjection() {
+
+		org.bson.Document mappedFields = mapper.getMappedFields(new org.bson.Document("id", 1), context.getPersistentEntity(WithStringId.class));
+		assertThat(mappedFields).containsEntry("_id", 1);
+	}
+
 	class WithDeepArrayNesting {
 
 		List<WithNestedArray> level0;
@@ -1367,6 +1375,12 @@ class Sample {
 		@Id private String foo;
 	}
 
+	class WithStringId {
+
+		@MongoId String id;
+		String name;
+	}
+
 	class BigIntegerId {
 
 		@Id private BigInteger id;