Fixed #22847: Optimizer wasn't expecting unresolved FKs

from __future__ import unicode_literals

from django.db import migrations

from django.utils import six

class MigrationOptimizer(object):

            # Don't allow optimisations of FKs through models they reference

            if hasattr(other.field, "rel") and other.field.rel:

                for between in in_between:

                    if between.references_model(

                        other.field.rel.to._meta.object_name,

                        other.field.rel.to._meta.app_label,

                    ):

                    if isinstance(other.field.rel.to, six.string_types):

                        object_name, app_label = other.field.rel.to.split(".", 1)

                    else:

                        object_name = other.field.rel.to._meta.object_name

                        app_label = other.field.rel.to._meta.app_label

                    if between.references_model(object_name, app_label):

                        return None

            # OK, that's fine

            return [