Use _prefetch_related_lookups and refine test cases

rest_framework/generics.py

@@ -1,8 +1,6 @@
 """
 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
@@ -47,8 +45,6 @@ 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.
@@ -72,31 +68,10 @@ 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):
         """
         Returns the object the view is displaying.

rest_framework/mixins.py

@@ -74,7 +74,8 @@ class UpdateModelMixin:
             # 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())
+            queryset = self.filter_queryset(self.get_queryset())
+            prefetch_related_objects([instance], *queryset._prefetch_related_lookups)
 
         return Response(serializer.data)
 

tests/test_prefetch_related.py

@@ -8,36 +8,52 @@ factory = APIRequestFactory()
 
 
 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 UserUpdate(generics.UpdateAPIView):
+    queryset = User.objects.exclude(username='exclude').prefetch_related('groups__permissions')
+    serializer_class = UserSerializer
+
+
+class UserUpdateWithoutPrefetchRelated(generics.UpdateAPIView):
     queryset = User.objects.exclude(username='exclude')
     serializer_class = UserSerializer
-    prefetch_related = ['groups']
 
 
 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.expected = {
             'id': self.user.pk,
-            'username': 'new',
-            'groups': [1],
+            'username': 'tom',
+            'groups': [group.pk for group in self.groups],
             'email': 'tom@example.com',
+            'permissions': [],
         }
         self.view = UserUpdate.as_view()
 
     def test_prefetch_related_updates(self):
+        self.groups.append(Group.objects.create(name='c'))
         request = factory.put(
-            '/', {'username': 'new', 'groups': [self.groups[0].pk]}, format='json'
+            '/', {'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() == 1
+        assert User.objects.get(pk=self.user.pk).groups.count() == 11
         assert response.data == self.expected
 
     def test_prefetch_related_excluding_instance_from_original_queryset(self):
@@ -50,4 +66,18 @@ class TestPrefetchRelatedUpdates(TestCase):
         response = self.view(request, pk=self.user.pk)
         assert User.objects.get(pk=self.user.pk).groups.count() == 1
         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(15):
+            UserUpdateWithoutPrefetchRelated.as_view()(request, pk=self.user.pk)