Support @DocumentReference via Querydsl.

Closes #4037
Original pull request: #4069.

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java

@@ -22,7 +22,6 @@
 import java.util.regex.Pattern;
 
 import org.bson.Document;
-
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -158,8 +157,20 @@ protected Object convert(@Nullable Path<?> path, @Nullable Constant<?> constant)
 
 		MongoPersistentProperty property = getPropertyFor(path);
 
-		return property.isIdProperty() ? asReference(constant.getConstant(), path.getMetadata().getParent())
-				: asReference(constant.getConstant(), path);
+		if (property.isDocumentReference()) {
+			return converter.toDocumentPointer(constant.getConstant(), property).getPointer();
+		}
+
+		if (property.isIdProperty()) {
+
+			MongoPersistentProperty propertyForPotentialDbRef = getPropertyForPotentialDbRef(path);
+			if (propertyForPotentialDbRef != null && propertyForPotentialDbRef.isDocumentReference()) {
+				return converter.toDocumentPointer(constant.getConstant(), propertyForPotentialDbRef).getPointer();
+			}
+			return asReference(constant.getConstant(), path.getMetadata().getParent());
+		}
+
+		return asReference(constant.getConstant(), path);
 	}
 
 	@Nullable

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializerUnitTests.java

@@ -50,6 +50,7 @@
 import com.querydsl.core.types.dsl.PathBuilder;
 import com.querydsl.core.types.dsl.SimplePath;
 import com.querydsl.core.types.dsl.StringPath;
+import org.springframework.data.mongodb.repository.User;
 
 /**
  * Unit tests for {@link SpringDataMongodbSerializer}.
@@ -225,6 +226,26 @@ void chainMultipleAndFlattensCorrectly() {
 		assertThat(serializer.handle(testExpression)).isEqualTo(expected);
 	}
 
+	@Test // GH-4037
+	void parsesDocumentReference() {
+
+		User user = new User();
+		user.setId("007");
+		Predicate predicate = QPerson.person.spiritAnimal.eq(user);
+
+		assertThat(serializer.handle(predicate)).isEqualTo(Document.parse("{ 'spiritAnimal' : '007' }"));
+	}
+
+	@Test // GH-4037
+	void parsesDocumentReferenceOnId() {
+
+		User user = new User();
+		user.setId("007");
+		Predicate predicate = QPerson.person.spiritAnimal.id.eq("007");
+
+		assertThat(serializer.handle(predicate)).isEqualTo(Document.parse("{ 'spiritAnimal' : '007' }"));
+	}
+
 	class Address {
 		String id;
 		String street;