def get(self, object_id, cls=None):
        """
        Retrieve an object by its object_id

        :param: object_id: the objects id.
        :param: cls: the objects class, if None use the default class from the datamanager
        :return: the requested object or None if not found
        """
        cls = self.cls if cls is None else cls
        return self.session.query(cls).get(object_id)