Fixed #25129 -- Made model instance defaults work with migrations (refs #24919).

because they cannot be :ref:`serialized by migrations <migration-serializing>`.

See that documentation for other caveats.

For fields like :class:`ForeignKey` that map to model instances, defaults

should be the value of the field they reference (``pk`` unless

:attr:`~ForeignKey.to_field` is set) instead of model instances.

The default value is used when new model instances are created and a value

isn't provided for the field. When the field is a primary key, the default is

also used when the field is set to ``None``.

def get_default_r():

    return R.objects.get_or_create(is_default=True)[0]

    return R.objects.get_or_create(is_default=True)[0].pk

class S(models.Model):

        a = create_a('setvalue')

        a.setvalue.delete()

        a = A.objects.get(pk=a.pk)

        self.assertEqual(self.DEFAULT, a.setvalue)

        self.assertEqual(self.DEFAULT, a.setvalue.pk)

    def test_setnull(self):

        a = create_a('setnull')

        a = create_a('setdefault')

        a.setdefault.delete()

        a = A.objects.get(pk=a.pk)

        self.assertEqual(self.DEFAULT, a.setdefault)

        self.assertEqual(self.DEFAULT, a.setdefault.pk)

    def test_setdefault_none(self):

        a = create_a('setdefault_none')

def choice_default():

    return ChoiceOptionModel.objects.get_or_create(name='default')[0]

    return ChoiceOptionModel.objects.get_or_create(name='default')[0].pk

def choice_default_list():

def get_foo():

    return Foo.objects.get(id=1)

    return Foo.objects.get(id=1).pk

class Bar(models.Model):