class StringConfReference(object):

    def __init__(self, name):
        self.name = name

def __str__(self):
    return getattr(conf, self.name)

def deconstruct(self):
    return '%s.%s' % (__name__, self.__class__.__name__), (self.name,), {}
attachment = models.FilePathField(path=StringConfReference('ATTACHMENT_DIR'
    ), recursive=True)
Миграции Django и настраиваемые многоразовые приложения