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

Backport of 6afa6818fcf25665bbf61f0921c8c8c6fa8f223e from master

3 files changed, 28 insertions, 2 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index ac8bd1c860..938535bf6a 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -1102,7 +1102,9 @@ class ModelChoiceIterator(object):
     def __iter__(self):
         if self.field.empty_label is not None:
             yield ("", self.field.empty_label)
-        for obj in self.queryset.iterator():
+        # Can't use iterator() when queryset uses prefetch_related()
+        method = 'all' if self.queryset._prefetch_related_lookups else 'iterator'
+        for obj in getattr(self.queryset, method)():
             yield self.choice(obj)
 
     def __len__(self):
diff --git a/docs/releases/1.8.6.txt b/docs/releases/1.8.6.txt
index 6268bb3bbf..9292d7835b 100644
--- a/docs/releases/1.8.6.txt
+++ b/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`).
diff --git a/tests/model_forms/tests.py b/tests/model_forms/tests.py
index 98b9c1ad52..12401759bd 100644
--- a/tests/model_forms/tests.py
+++ b/tests/model_forms/tests.py
@@ -2285,6 +2285,29 @@ 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')