Fix UpdateModelMixin to work when no queryset is defined

Fix #9306

Author: browniebroke

rest_framework/mixins.py

@@ -4,7 +4,7 @@
 We don't bind behaviour to http method handlers yet,
 which allows mixin classes to be composed in interesting ways.
 """
-from django.db.models.query import prefetch_related_objects
+from django.db.models.query import prefetch_related_objects, QuerySet
 
 from rest_framework import status
 from rest_framework.response import Response
@@ -69,13 +69,14 @@ def update(self, request, *args, **kwargs):
         serializer.is_valid(raise_exception=True)
         self.perform_update(serializer)
 
-        queryset = self.filter_queryset(self.get_queryset())
-        if queryset._prefetch_related_lookups:
+        if hasattr(instance, '_prefetched_objects_cache'):
             # If 'prefetch_related' has been applied to a queryset, we need to
-            # forcibly invalidate the prefetch cache on the instance,
-            # and then re-prefetch related objects
+            # forcibly invalidate the prefetch cache on the instance
             instance._prefetched_objects_cache = {}
-            prefetch_related_objects([instance], *queryset._prefetch_related_lookups)
+            queryset = self.filter_queryset(self.get_queryset())
+            if getattr(queryset, '_prefetch_related_lookups', None):
+                # And then re-prefetch related objects
+                prefetch_related_objects([instance], *queryset._prefetch_related_lookups)
 
         return Response(serializer.data)
 

tests/test_prefetch_related.py

@@ -35,6 +35,13 @@ class UserUpdateWithoutPrefetchRelated(generics.UpdateAPIView):
     serializer_class = UserSerializer
 
 
+class UserRetrieveWithoutQuerySet(generics.RetrieveUpdateAPIView):
+    serializer_class = UserSerializer
+
+    def get_object(self):
+        return User.objects.get(pk=self.kwargs['pk'])
+
+
 class TestPrefetchRelatedUpdates(TestCase):
     def setUp(self):
         self.user = User.objects.create(username='tom', email='[email protected]')
@@ -90,3 +97,11 @@ def test_db_query_count(self):
         )
         with self.assertNumQueries(16):
             UserUpdateWithoutPrefetchRelated.as_view()(request, pk=self.user.pk)
+
+    def test_can_update_without_queryset(self):
+        request = factory.patch('/', {'username': 'new'})
+        response = UserRetrieveWithoutQuerySet.as_view()(request, pk=self.user.pk)
+        assert response.data['id'] == self.user.id
+        assert response.data['username'] == 'new'
+        self.user.refresh_from_db()
+        assert self.user.username == 'new'