Get all attrs necessary for the update (empty unless Create)

def get_attrs(self):
        """ Get all attrs necessary for the update (empty unless Create) """
        if self.action != 'Create':
            return []
        index = self.extra['index']
        ret = [index.hash_key]
        if index.range_key is not None:
            ret.append(index.range_key)
        return ret