[1.8.x] Fixed #25683 -- Allowed ModelChoiceField(queryset=...) to accept Managers.

This fixes a regression from refs #25496.

Backport of 1155843a41 from master

django/forms/models.py

@@ -1092,17 +1092,19 @@ class ModelChoiceIterator(object):
     def __iter__(self):
         if self.field.empty_label is not None:
             yield ("", self.field.empty_label)
-        method = 'all' if self.queryset._prefetch_related_lookups else 'iterator'
+        queryset = self.queryset.all()
-        queryset = getattr(self.queryset, method)
+        # Can't use iterator() when queryset uses prefetch_related()
+        if not queryset._prefetch_related_lookups:
+            queryset = queryset.iterator()
         if self.field.cache_choices:
             if self.field.choice_cache is None:
                 self.field.choice_cache = [
-                    self.choice(obj) for obj in queryset()
+                    self.choice(obj) for obj in queryset
                 ]
             for choice in self.field.choice_cache:
                 yield choice
         else:
-            for obj in queryset():
+            for obj in queryset:
                 yield self.choice(obj)
 
     def __len__(self):

docs/releases/1.8.7.txt

@@ -14,3 +14,7 @@ Bugfixes
 
 * Fixed a regression in 1.8.6 that caused database routers without an
   ``allow_migrate()`` method to crash (:ticket:`25686`).
+
+* Fixed a regression in 1.8.6 by restoring the ability to use ``Manager``
+  objects for the ``queryset`` argument of ``ModelChoiceField``
+  (:ticket:`25683`).

tests/forms_tests/tests/tests.py

@@ -60,17 +60,23 @@ class FileForm(Form):
     file1 = FileField()
 
 
-class TestTicket12510(TestCase):
+class TestModelChoiceField(TestCase):
-    ''' It is not necessary to generate choices for ModelChoiceField (regression test for #12510). '''
-    def setUp(self):
-        self.groups = [Group.objects.create(name=name) for name in 'abc']
 
     def test_choices_not_fetched_when_not_rendering(self):
+        """
+        Generating choices for ModelChoiceField should require 1 query (#12510).
+        """
+        self.groups = [Group.objects.create(name=name) for name in 'abc']
         # only one query is required to pull the model from DB
         with self.assertNumQueries(1):
             field = ModelChoiceField(Group.objects.order_by('-name'))
             self.assertEqual('a', field.clean(self.groups[0].pk).name)
 
+    def test_queryset_manager(self):
+        f = ModelChoiceField(ChoiceOptionModel.objects)
+        choice = ChoiceOptionModel.objects.create(name="choice 1")
+        self.assertEqual(list(f.choices), [('', '---------'), (choice.pk, str(choice))])
+
 
 class TestTicket14567(TestCase):
     """