Using the 'loaddata' management command in a data migration with South is going to break things as soon as the used models change in any way (when running the migrations on a fresh install). The data loaders don't know about the frozen ORM provided by South but use the current state of the models as in the respective 'models.py'. Here's a fixtur…

0002_datamigration.py

from myproject.utils import loaddata


class Migration(DataMigration):

    def forwards(self, orm):

        loaddata(orm, 'some_fancy_fixture.json')

utils.py

def loaddata(orm, fixture_name):
    """
    Overwrite the ``_get_model`` command in the serialiser to use the
    FakeORM model from south instead of the latest model.
    """
    from dingus import patch

    _get_model = lambda model_identifier: orm[model_identifier]

    with patch('django.core.serializers.python._get_model', _get_model):
        from django.core.management import call_command
        call_command("loaddata", fixture_name)

Thanks for this workaround!