@classmethod
def one(cls, *args):
    """Get one resource."""
    rv = None
    if cls.__key__ == ('id',):
        msg = 'The keys does not ensure the uniqueness of an instance.'
        query = 'SELECT id, obj FROM %s WHERE id = ?' % cls.__tablename__
        c = cls._session().cursor()
        for row in c.execute(query, args):
            if rv is not None:
                raise NotImplementedError(msg)
            rv = cls._deserialize(*row)
    else:
        rv = super().one(*args)
    return rv
