[1.8.x] Fixed #25496 -- Made ModelChoiceField respect prefetch_related().

Backport of 6afa6818fc from master

django/forms/models.py

@@ -1092,15 +1092,17 @@ 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 = getattr(self.queryset, method)
         if self.field.cache_choices:
             if self.field.choice_cache is None:
                 self.field.choice_cache = [
-                    self.choice(obj) for obj in self.queryset.iterator()
+                    self.choice(obj) for obj in queryset()
                 ]
             for choice in self.field.choice_cache:
                 yield choice
         else:
-            for obj in self.queryset.iterator():
+            for obj in queryset():
                 yield self.choice(obj)
 
     def __len__(self):

docs/releases/1.8.6.txt

@@ -9,4 +9,5 @@ Django 1.8.6 fixes several bugs in 1.8.5.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression causing ``ModelChoiceField`` to ignore
+  ``prefetch_related()`` on its queryset (:ticket:`25496`).

tests/model_forms/tests.py

@@ -2203,6 +2203,29 @@ class OtherModelFormTests(TestCase):
         with self.assertRaises(ValidationError):
             f.fields['status'].clean('z')
 
+    def test_prefetch_related_queryset(self):
+        """
+        ModelChoiceField should respect a prefetch_related() on its queryset.
+        """
+        blue = Colour.objects.create(name='blue')
+        red = Colour.objects.create(name='red')
+        multicolor_item = ColourfulItem.objects.create()
+        multicolor_item.colours.add(blue, red)
+        red_item = ColourfulItem.objects.create()
+        red_item.colours.add(red)
+
+        class ColorModelChoiceField(forms.ModelChoiceField):
+            def label_from_instance(self, obj):
+                return ', '.join(c.name for c in obj.colours.all())
+
+        field = ColorModelChoiceField(ColourfulItem.objects.prefetch_related('colours'))
+        with self.assertNumQueries(4):  # would be 5 if prefetch is ignored
+            self.assertEqual(tuple(field.choices), (
+                ('', '---------'),
+                (multicolor_item.pk, 'blue, red'),
+                (red_item.pk, 'red'),
+            ))
+
     def test_foreignkeys_which_use_to_field(self):
         apple = Inventory.objects.create(barcode=86, name='Apple')
         Inventory.objects.create(barcode=22, name='Pear')