Re-prefetch related objects after updating (#8043)

* Re-prefetch related objects after updating

* Fix flake8 format

* Use _prefetch_related_lookups and refine test cases

* Add more test cases and refine prefetch checking

Author: yuekui

rest_framework/mixins.py

@@ -4,6 +4,8 @@
 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
@@ -67,10 +69,13 @@ def update(self, request, *args, **kwargs):
         serializer.is_valid(raise_exception=True)
         self.perform_update(serializer)
 
-        if getattr(instance, '_prefetched_objects_cache', None):
+        queryset = self.filter_queryset(self.get_queryset())
+        if queryset._prefetch_related_lookups:
             # 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], *queryset._prefetch_related_lookups)
 
         return Response(serializer.data)
 

tests/test_prefetch_related.py

@@ -1,4 +1,5 @@
 from django.contrib.auth.models import Group, User
+from django.db.models.query import Prefetch
 from django.test import TestCase
 
 from rest_framework import generics, serializers
@@ -8,51 +9,84 @@
 
 
 class UserSerializer(serializers.ModelSerializer):
+    permissions = serializers.SerializerMethodField()
+
+    def get_permissions(self, obj):
+        ret = []
+        for g in obj.groups.all():
+            ret.extend([p.pk for p in g.permissions.all()])
+        return ret
+
     class Meta:
         model = User
-        fields = ('id', 'username', 'email', 'groups')
+        fields = ('id', 'username', 'email', 'groups', 'permissions')
+
+
+class UserRetrieveUpdate(generics.RetrieveUpdateAPIView):
+    queryset = User.objects.exclude(username='exclude').prefetch_related(
+        Prefetch('groups', queryset=Group.objects.exclude(name='exclude')),
+        'groups__permissions',
+    )
+    serializer_class = UserSerializer
 
 
-class UserUpdate(generics.UpdateAPIView):
-    queryset = User.objects.exclude(username='exclude').prefetch_related('groups')
+class UserUpdateWithoutPrefetchRelated(generics.UpdateAPIView):
+    queryset = User.objects.exclude(username='exclude')
     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.groups = [Group.objects.create(name=f'group {i}') for i in range(10)]
         self.user.groups.set(self.groups)
+        self.user.groups.add(Group.objects.create(name='exclude'))
+        self.expected = {
+            'id': self.user.pk,
+            'username': 'tom',
+            'groups': [group.pk for group in self.groups],
+            'email': 'tom@example.com',
+            'permissions': [],
+        }
+        self.view = UserRetrieveUpdate.as_view()
 
     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
+        self.groups.append(Group.objects.create(name='c'))
+        request = factory.put(
+            '/', {'username': 'new', 'groups': [group.pk for group in self.groups]}, format='json'
+        )
+        self.expected['username'] = 'new'
+        self.expected['groups'] = [group.pk for group in self.groups]
+        response = self.view(request, pk=self.user.pk)
+        assert User.objects.get(pk=self.user.pk).groups.count() == 12
+        assert response.data == self.expected
+        # Update and fetch should get same result
+        request = factory.get('/')
+        response = self.view(request, pk=self.user.pk)
+        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() == 2
+        self.expected['username'] = 'exclude'
+        self.expected['groups'] = [self.groups[0].pk]
+        assert response.data == self.expected
+
+    def test_db_query_count(self):
+        request = factory.put(
+            '/', {'username': 'new'}, format='json'
+        )
+        with self.assertNumQueries(7):
+            self.view(request, pk=self.user.pk)
+
+        request = factory.put(
+            '/', {'username': 'new2'}, format='json'
+        )
+        with self.assertNumQueries(16):
+            UserUpdateWithoutPrefetchRelated.as_view()(request, pk=self.user.pk)