def query_delete(cls, expression=None):
        """
        Delete model instances matching the given expression (if
        specified). If no expression is provided, then all model instances
        will be deleted.

        :param expression: A boolean expression to filter by.
        """
        if expression is not None:
            executor = Executor(cls.__database__)
            result = executor.execute(expression)
        else:
            result = cls._query.all_index()

        for hash_id in result:
            cls.load(hash_id, convert_key=False).delete()