[1.11.x] Fixed #28096 -- Allowed prefetch calls with ModelIterable subclasses

Regression in 7ec330eeb9.
Thanks Tim Graham for the review.
Backport of 43b4a1618e from master.

django/db/models/query.py

@@ -1305,7 +1305,7 @@ 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:
+        if queryset is not None and not issubclass(queryset._iterable_class, ModelIterable):
             raise ValueError('Prefetch querysets cannot use values().')
         if to_attr:
             self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])

docs/releases/1.11.1.txt

@@ -33,3 +33,6 @@ Bugfixes
 
 * Fixed layout of ``ReadOnlyPasswordHashWidget`` (used in the admin's user
   change page) (:ticket:`28097`).
+
+* Allowed prefetch calls on managers with custom ``ModelIterable`` subclasses
+  (:ticket:`28096`).

tests/prefetch_related/models.py

@@ -5,6 +5,7 @@ from django.contrib.contenttypes.fields import (
 )
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
+from django.db.models.query import ModelIterable, QuerySet
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.functional import cached_property
 
@@ -100,6 +101,16 @@ class Qualification(models.Model):
         ordering = ['id']
 
 
+class ModelIterableSubclass(ModelIterable):
+    pass
+
+
+class TeacherQuerySet(QuerySet):
+    def __init__(self, *args, **kwargs):
+        super(TeacherQuerySet, self).__init__(*args, **kwargs)
+        self._iterable_class = ModelIterableSubclass
+
+
 class TeacherManager(models.Manager):
     def get_queryset(self):
         return super(TeacherManager, self).get_queryset().prefetch_related('qualifications')
@@ -111,6 +122,7 @@ class Teacher(models.Model):
     qualifications = models.ManyToManyField(Qualification)
 
     objects = TeacherManager()
+    objects_custom = TeacherQuerySet.as_manager()
 
     def __str__(self):
         return "%s (%s)" % (self.name, ", ".join(q.name for q in self.qualifications.all()))

tests/prefetch_related/tests.py

@@ -15,8 +15,8 @@ from django.utils.encoding import force_text
 from .models import (
     Author, Author2, AuthorAddress, AuthorWithAge, Bio, Book, Bookmark,
     BookReview, BookWithYear, Comment, Department, Employee, FavoriteAuthors,
-    House, LessonEntry, Person, Qualification, Reader, Room, TaggedItem,
-    Teacher, WordEntry,
+    House, LessonEntry, ModelIterableSubclass, Person, Qualification, Reader,
+    Room, TaggedItem, Teacher, WordEntry,
 )
 
 
@@ -738,6 +738,9 @@ class CustomPrefetchTests(TestCase):
     def test_values_queryset(self):
         with self.assertRaisesMessage(ValueError, 'Prefetch querysets cannot use values().'):
             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_to_attr_doesnt_cache_through_attr_as_list(self):
         house = House.objects.prefetch_related(