Set the ancestor for the query

        :type value: :class:`~google.cloud.datastore.key.Key`
        :param value: the new ancestor key

def ancestor(self, value):
        """Set the ancestor for the query

        :type value: :class:`~google.cloud.datastore.key.Key`
        :param value: the new ancestor key
        """
        if not isinstance(value, Key):
            raise TypeError("Ancestor must be a Key")
        self._ancestor = value