accessing model manager methods in datamigration

Looks like this is not possible.

From the South documentation:

You can do a lot more with this inside a data migration; any model can be available to you. The only caveat is that you won’t have access to any custom methods or managers on your models, as they’re not preserved as part of the freezing process (there’s no way to do this generally); you’ll have to copy any code you want into the migration itself. Feel free to make them methods on the Migration class; South ignores everything apart from forwards and backwards.

Since Django 1.8, you can include model managers in migrations by adding the use_in_migrations property.

From the docs: https://docs.djangoproject.com/en/2.0/topics/migrations/#model-managers

class MyManager(models.Manager):
    use_in_migrations = True

class MyModel(models.Model):
    objects = MyManager()