Fixed #21008 -- Made MigrationWriter handle Promise objects.

from django.db import models

from django.db.models.loading import cache

from django.db.migrations.loader import MigrationLoader

from django.utils.encoding import force_text

from django.utils.functional import Promise

class MigrationWriter(object):

        # Simple types

        elif isinstance(value, six.integer_types + (float, six.binary_type, six.text_type, bool, type(None))):

            return repr(value), set()

        # Promise

        elif isinstance(value, Promise):

            return repr(force_text(value)), set()

        # Django fields

        elif isinstance(value, models.Field):

            attr_name, path, args, kwargs = value.deconstruct()

from django.test import TestCase

from django.db.migrations.writer import MigrationWriter

from django.db import models, migrations

from django.utils.translation import ugettext_lazy as _

class WriterTests(TestCase):

        self.assertSerializedEqual(["a", 2, True, None])

        self.assertSerializedEqual(set([2, 3, "eighty"]))

        self.assertSerializedEqual({"lalalala": ["yeah", "no", "maybe"]})

        self.assertSerializedEqual(_('Hello'))

        # Datetime stuff

        self.assertSerializedEqual(datetime.datetime.utcnow())

        self.assertSerializedEqual(datetime.datetime.utcnow)