嵌套动态密钥的安全规则通配符

Question

我是Firebase的新手，我对安全规则感到困惑。我有一个像这样的在线用户数据库：

USER_ONLINE_STATUS:
    ---- channelOneId (dynamic)
            ---- user1Id (dynamic): "ONLINE"
            ---- user2Id (dynamic): "AWAY"
    ---- channelTwoId (dynamic)
            ---- user1Id (dynamic): "OFFLINE"
            ---- user2Id (dynamic): "ONLINE"

我尝试的是-

{
  "rules": {
    "USER_ONLINE_STATUS": {
        "$channelId": {
            ".read": "true",
            "$user_id": {
                 ".write": "auth.uid === $user_id",
                 ".validate": "newData.isString() && newData.val().length > 0 && newData.val().length < 20"
            }
        }
     }  
  }
}

但这给我设置了/ USER_ONLINE_STATUS / c5ba55dfc11557daaaf7ppllmm34 / 5ecfa107wpwpwpwp393f78f024失败的错误：Permission_denied