Fixed #31044 -- Errored nicely when using Prefetch with a raw() queryset.

django/db/models/query.py

@@ -1534,8 +1534,15 @@ class Prefetch:
         self.prefetch_through = lookup
         # `prefetch_to` is the path to the attribute that stores the result.
         self.prefetch_to = lookup
-        if queryset is not None and not issubclass(queryset._iterable_class, ModelIterable):
-            raise ValueError('Prefetch querysets cannot use values().')
+        if queryset is not None and (
+            isinstance(queryset, RawQuerySet) or (
+                hasattr(queryset, '_iterable_class') and
+                not issubclass(queryset._iterable_class, ModelIterable)
+            )
+        ):
+            raise ValueError(
+                'Prefetch querysets cannot use raw() and values().'
+            )
         if to_attr:
             self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])
 

tests/prefetch_related/tests.py

@@ -815,12 +815,18 @@ class CustomPrefetchTests(TestCase):
             self.traverse_qs(list(houses), [['occupants', 'houses', 'main_room']])
 
     def test_values_queryset(self):
-        with self.assertRaisesMessage(ValueError, 'Prefetch querysets cannot use values().'):
+        msg = 'Prefetch querysets cannot use raw() and values().'
+        with self.assertRaisesMessage(ValueError, msg):
             Prefetch('houses', House.objects.values('pk'))
         # That error doesn't affect managers with custom ModelIterable subclasses
         self.assertIs(Teacher.objects_custom.all()._iterable_class, ModelIterableSubclass)
         Prefetch('teachers', Teacher.objects_custom.all())
 
+    def test_raw_queryset(self):
+        msg = 'Prefetch querysets cannot use raw() and values().'
+        with self.assertRaisesMessage(ValueError, msg):
+            Prefetch('houses', House.objects.raw('select pk from house'))
+
     def test_to_attr_doesnt_cache_through_attr_as_list(self):
         house = House.objects.prefetch_related(
             Prefetch('rooms', queryset=Room.objects.all(), to_attr='to_rooms'),