Fix return stale data on PATCH to-one relationship (#247)

Author: tpilara

example/tests/test_views.py

@@ -101,6 +101,7 @@ def test_patch_to_one_relationship(self):
         }
         response = self.client.patch(url, data=json.dumps(request_data), content_type='application/vnd.api+json')
         assert response.status_code == 200, response.content.decode()
+        assert response.data == request_data['data']
 
         response = self.client.get(url)
         assert response.data == request_data['data']
@@ -112,6 +113,7 @@ def test_patch_to_many_relationship(self):
         }
         response = self.client.patch(url, data=json.dumps(request_data), content_type='application/vnd.api+json')
         assert response.status_code == 200, response.content.decode()
+        assert response.data == request_data['data']
 
         response = self.client.get(url)
         assert response.data == request_data['data']

rest_framework_json_api/views.py

@@ -99,6 +99,7 @@ def patch(self, request, *args, **kwargs):
             serializer.is_valid(raise_exception=True)
             setattr(parent_obj, self.get_related_field_name(), serializer.validated_data)
             parent_obj.save()
+            related_instance_or_manager = self.get_related_instance()  # Refresh instance
         result_serializer = self._instantiate_serializer(related_instance_or_manager)
         return Response(result_serializer.data)