Fixed #25496 -- Made ModelChoiceField respect prefetch_related().

    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):

Bugfixes

========

* ...

* Fixed a regression causing ``ModelChoiceField`` to ignore

  ``prefetch_related()`` on its queryset (:ticket:`25496`).

        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')