Re-prefetch related objects after updating

rest_framework/generics.py

@@ -1,6 +1,7 @@
 """
 Generic views that provide commonly needed behaviour.
 """
+from typing import Iterable
 from django.core.exceptions import ValidationError
 from django.db.models.query import QuerySet
 from django.http import Http404
@@ -45,6 +46,8 @@ class GenericAPIView(views.APIView):
     # The style to use for queryset pagination.
     pagination_class = api_settings.DEFAULT_PAGINATION_CLASS
 
+    prefetch_related = []
+
     def get_queryset(self):
         """
         Get the list of items for this view.
@@ -68,9 +71,30 @@ class GenericAPIView(views.APIView):
 
         queryset = self.queryset
         if isinstance(queryset, QuerySet):
+            # Prefetch related objects
+            if self.get_prefetch_related():
+                queryset = queryset.prefetch_related(*self.get_prefetch_related())
             # Ensure queryset is re-evaluated on each request.
             queryset = queryset.all()
         return queryset
+    
+    def get_prefetch_related(self):
+        """
+        Get the list of prefetch related objects for self.queryset or instance.
+        This must be an iterable.
+        Defaults to using `self.prefetch_related`.
+
+        You may want to override this if you need to provide prefetched objects
+        depending on the incoming request.
+
+        (Eg. `['toppings', Prefetch('restaurants', queryset=Restaurant.objects.select_related('best_pizza'))]`)
+        """
+        assert isinstance(self.prefetch_related, Iterable), (
+            "'%s' should either include an iterable `prefetch_related` attribute, "
+            "or override the `get_prefetch_related()` method."
+            % self.__class__.__name__
+        )
+        return self.prefetch_related
 
     def get_object(self):
         """

rest_framework/mixins.py

@@ -4,6 +4,8 @@ Basic building blocks for generic class based views.
 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 rest_framework import status
 from rest_framework.response import Response
 from rest_framework.settings import api_settings
@@ -69,8 +71,10 @@ class UpdateModelMixin:
 
         if getattr(instance, '_prefetched_objects_cache', None):
             # If 'prefetch_related' has been applied to a queryset, we need to
-            # forcibly invalidate the prefetch cache on the instance.
+            # forcibly invalidate the prefetch cache on the instance,
+            # and then re-prefetch related objects
             instance._prefetched_objects_cache = {}
+            prefetch_related_objects([instance], *self.get_prefetch_related())
 
         return Response(serializer.data)
 

tests/test_prefetch_related.py

@@ -14,8 +14,9 @@ class UserSerializer(serializers.ModelSerializer):
 
 
 class UserUpdate(generics.UpdateAPIView):
-    queryset = User.objects.exclude(username='exclude').prefetch_related('groups')
+    queryset = User.objects.exclude(username='exclude')
     serializer_class = UserSerializer
+    prefetch_related = ['groups']
 
 
 class TestPrefetchRelatedUpdates(TestCase):
@@ -23,36 +24,30 @@ class TestPrefetchRelatedUpdates(TestCase):
         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.set(self.groups)
-
-    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,
+        self.expected = {
+            'id': self.user.pk,
             'username': 'new',
             'groups': [1],
-            'email': 'tom@example.com'
+            'email': 'tom@example.com',
         }
-        assert response.data == expected
+        self.view = UserUpdate.as_view()
+
+    def test_prefetch_related_updates(self):
+        request = factory.put(
+            '/', {'username': 'new', 'groups': [self.groups[0].pk]}, format='json'
+        )
+        response = self.view(request, pk=self.user.pk)
+        assert User.objects.get(pk=self.user.pk).groups.count() == 1
+        assert response.data == self.expected
 
     def test_prefetch_related_excluding_instance_from_original_queryset(self):
         """
         Regression test for https://github.com/encode/django-rest-framework/issues/4661
         """
-        view = UserUpdate.as_view()
-        pk = self.user.pk
-        groups_pk = self.groups[0].pk
-        request = factory.put('/', {'username': 'exclude', 'groups': [groups_pk]}, format='json')
-        response = view(request, pk=pk)
-        assert User.objects.get(pk=pk).groups.count() == 1
-        expected = {
-            'id': pk,
-            'username': 'exclude',
-            'groups': [1],
-            'email': 'tom@example.com'
-        }
-        assert response.data == expected
+        request = factory.put(
+            '/', {'username': 'exclude', 'groups': [self.groups[0].pk]}, format='json'
+        )
+        response = self.view(request, pk=self.user.pk)
+        assert User.objects.get(pk=self.user.pk).groups.count() == 1
+        self.expected['username'] = 'exclude'
+        assert response.data == self.expected