def delete(cls, object_version, key=None):
        """Delete tags.

        :param object_version: The object version instance or id.
        :param key: Key of the tag to delete.
            Default: delete all tags.
        """
        with db.session.begin_nested():
            q = cls.query.filter_by(
                version_id=as_object_version_id(object_version))
            if key:
                q = q.filter_by(key=key)
            q.delete()