Use setattr for adding fields to a new instance

Add test for restoring a GenericForeignKey

rest_framework/serializers.py

@@ -955,18 +955,16 @@ class ModelSerializer(Serializer):
             if isinstance(self.fields.get(field_name, None), Serializer):
                 nested_forward_relations[field_name] = attrs[field_name]
 
-        # Update an existing instance...
+        # Create an empty instance of the model
-        if instance is not None:
+        if instance is None:
+            instance = self.opts.model()
+
         for key, val in attrs.items():
             try:
                 setattr(instance, key, val)
             except ValueError:
                 self._errors[key] = self.error_messages['required']
 
-        # ...or create a new instance
-        else:
-            instance = self.opts.model(**attrs)
-
         # Any relations that cannot be set until we've
         # saved the model get hidden away on these
         # private attributes, so we can deal with them

rest_framework/tests/test_genericrelations.py

@@ -131,3 +131,21 @@ class TestGenericRelations(TestCase):
         }
         ]
         self.assertEqual(serializer.data, expected)
+
+    def test_restore_object_generic_fk(self):
+        """
+        Ensure an object with a generic foreign key can be restored.
+        """
+
+        class TagSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Tag
+                exclude = ('content_type', 'object_id')
+
+        serializer = TagSerializer()
+
+        bookmark = Bookmark(url='http://example.com')
+        attrs = {'tagged_item': bookmark, 'tag': 'example'}
+
+        tag = serializer.restore_object(attrs)
+        self.assertEqual(tag.tagged_item, bookmark)