'Indicate that user_id wishes to discard history for room_id.'

def forget(self, user_id, room_id):

    def f(txn):
        sql = 'UPDATE        room_memberships    SET        forgotten    =    1    WHERE        user_id    =    ?    AND        room_id    =    ?'
        txn.execute(sql, (user_id, room_id))
        txn.call_after(self.was_forgotten_at.invalidate_all)
        txn.call_after(self.did_forget.invalidate, (user_id, room_id))
        self._invalidate_cache_and_stream(txn, self.who_forgot_in_room, (room_id,))
    return self.runInteraction('forget_membership', f)
