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

Regression in 7ec330eeb96d0874949eacb8ed1bbb97e11807e1.
Thanks Tim Graham for the review.
Backport of 43b4a1618ed85151494d68778a1bb6304ce1cb79 from master.

2 files changed, 17 insertions, 2 deletions

diff --git a/tests/prefetch_related/models.py b/tests/prefetch_related/models.py
index 064ce1dfbd..6600418b8f 100644
--- a/tests/prefetch_related/models.py
+++ b/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()))
diff --git a/tests/prefetch_related/tests.py b/tests/prefetch_related/tests.py
index 7ddf4a4c96..23ea2458fc 100644
--- a/tests/prefetch_related/tests.py
+++ b/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(