Refs #26565 -- Errored nicely when using Prefetch with a values() queryset.

Thanks Maxime Lorant for the report and Anssi for the suggestion.

django/db/models/query.py

@@ -1331,6 +1331,8 @@ class Prefetch(object):
         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 queryset._iterable_class is not ModelIterable:
+            raise ValueError('Prefetch querysets cannot use values().')
         if to_attr:
             self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])
 

tests/prefetch_related/tests.py

@@ -733,6 +733,10 @@ class CustomPrefetchTests(TestCase):
         self.assertEqual(str(warns[0].message), msg)
         self.assertEqual(str(warns[0].message), msg)
 
+    def test_values_queryset(self):
+        with self.assertRaisesMessage(ValueError, 'Prefetch querysets cannot use values().'):
+            Prefetch('houses', House.objects.values('pk'))
+
 
 class DefaultManagerTests(TestCase):