Fix prefetching in DeferringManyRelatedManager._apply_rel_filters()

modelcluster/fields.py

@@ -397,7 +397,9 @@ def get_prefetch_queryset(self, instances, queryset=None):
 
         def _apply_rel_filters(self, queryset):
             # Required for get_prefetch_queryset.
-            return queryset._next_is_sticky()
+            # NOTE: _apply_rel_filters() must return a copy of the queryset
+            # to work correctly with prefetch
+            return queryset._next_is_sticky().all()
 
         def get_object_list(self):
             """

tests/tests/test_cluster.py

@@ -1311,3 +1311,19 @@ def test_prefetch_related_with_lookup(self):
         res = list(query)
         self.assertEqual(query[0].menu_items.all()[0], menu_item1)
         self.assertEqual(query[1].menu_items.all()[0], menu_item2)
+
+    def test_m2m_prefetch_related_with_lookup(self):
+        person1 = Person.objects.create(name='Joe')
+        person2 = Person.objects.create(name='Mary')
+        room = Room.objects.create(name='Dining room')
+        house = House.objects.create(name='House 1', address='123 Main St', owner=person1, main_room=room)
+        person1.houses = [house]
+        person1.save()
+
+        query = Person.objects.all().prefetch_related(
+            Prefetch('houses', queryset=House.objects.all())
+        )
+
+        res = list(query)
+        self.assertEqual(query[0], person1)
+        self.assertEqual(query[0].houses.count(), 1)