如何在节点中强制执行最大数量的子节点？

Question

如何使用安全规则强制执行节点的最大子节点数？

/bar/customers/$customer/drinks_ordered

不应该有超过十五个孩子.

Answer 1

看起来您可以使用数字ID来订购饮品,然后尝试这样的事情; 它将失败,因为ID是一个字符串:

"$customer_id": {
    "drinks_ordered": {
       "$drink_id": {
          ".validate": "$drink_id > 0 && $drink_id < 16" // error
       }
    }
}

相反,您可以使用计数器并将计数器验证为1-15,然后验证饮料ID是否与计数器匹配.

"$customer_id": {
    "counter": {
       // counter can only be incremented by 1 each time, must be a number
       // and must be <= 15
       ".validate": "newData.isNumber() && newData.val() > 0 && newData.val() <= 15 && ((!data.exists() && newData.val() === 1) || (newData.val() === data.val()+1))"
    },
    "drinks_ordered": {
       // new record's ID must match the incremented counter
       "$drink_id": {
          // use .val()+'' because $drink_id is a string and Firebase always uses ===!
          ".validate": "root.child('bar/customers/'+$customer_id+'/counter').val()+'' == $drink_id"
       }
    }
}

当然,你的饮料看起来像这样:

 /bar/customers/george_thorogood/counter/3
 /bar/customers/george_thorogood/drinks_ordered/1/burbon
 /bar/customers/george_thorogood/drinks_ordered/2/scotch
 /bar/customers/george_thorogood/drinks_ordered/3/beer

现在,在客户端可以添加另一种饮料之前,他们必须将计数器设置为4(这是唯一可以设置的),然后添加具有相同ID的饮料.

有点环形交叉口,但确实可以完成这项工作:)