Fix prefetch_related updates. (#4553)

rest_framework/mixins.py

@@ -68,6 +68,13 @@ class UpdateModelMixin(object):
         serializer = self.get_serializer(instance, data=request.data, partial=partial)
         serializer.is_valid(raise_exception=True)
         self.perform_update(serializer)
+
+        if getattr(instance, '_prefetched_objects_cache', None):
+            # If 'prefetch_related' has been applied to a queryset, we need to
+            # refresh the instance from the database.
+            instance = self.get_object()
+            serializer = self.get_serializer(instance)
+
         return Response(serializer.data)
 
     def perform_update(self, serializer):

tests/test_prefetch_related.py

+from django.contrib.auth.models import Group, User
+from django.test import TestCase
+
+from rest_framework import generics, serializers
+from rest_framework.test import APIRequestFactory
+
+factory = APIRequestFactory()
+
+
+class UserSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = User
+        fields = ('id', 'username', 'email', 'groups')
+
+
+class UserUpdate(generics.UpdateAPIView):
+    queryset = User.objects.all().prefetch_related('groups')
+    serializer_class = UserSerializer
+
+
+class TestPrefetchRelatedUpdates(TestCase):
+    def setUp(self):
+        self.user = User.objects.create(username='tom', email='tom@example.com')
+        self.groups = [Group.objects.create(name='a'), Group.objects.create(name='b')]
+        self.user.groups = self.groups
+        self.user.save()
+
+    def test_prefetch_related_updates(self):
+        view = UserUpdate.as_view()
+        pk = self.user.pk
+        groups_pk = self.groups[0].pk
+        request = factory.put('/', {'username': 'new', 'groups': [groups_pk]}, format='json')
+        response = view(request, pk=pk)
+        assert User.objects.get(pk=pk).groups.count() == 1
+        expected = {
+            'id': pk,
+            'username': 'new',
+            'groups': [1],
+            'email': 'tom@example.com'
+        }
+        assert response.data == expected