def create(cls, object_version, key, value):
        """Create a new tag for a given object version."""
        assert len(key) < 256
        assert len(value) < 256
        with db.session.begin_nested():
            obj = cls(version_id=as_object_version_id(object_version),
                      key=key,
                      value=value)
            db.session.add(obj)
        return obj