我如何在一个节点中执行最大数量的子节点？ [英] How can I enforce a maximum number of children in a node?

问题描述

  / bar / customers / $ 
 
如何使用安全规则强制执行一个节点的最大子节点数？ customer / drinks_ordered 
  

不应该超过15个孩子。

看起来好像你可以使用一个数字ID作为排序的饮料，然后尝试这样的事情;它会失败，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确认为1-15，然后

 $ customer_id：{
counter：{
 //计数器每次只能增加1，必须是数字
 //并且必须是< = 15 
.validate：newData.isNumber（）&& newData .val（）> 0& newData.val（）< = 15&&&（（！data.exists（）& newData.val（）=== 1）||（newData .val（）=== data.val（）+ 1））
，
drinks_ordered：{
 //新记录的ID必须匹配递增的计数器
 $ drink_id：{
 //使用.val（）+''，因为$ drink_id是一个st戒指和Firebase总是使用===！ 
.validate：root.child（'bar / customers /'+ $ customer_id +'/ counter'）。val（）+''== $ drink_id
} 
} 
 
  

自然地，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。 b
$ b

有点迂回，但它确实可以做到这一点）

How can I enforce a maximum number of children of a node using a security rule?

/bar/customers/$customer/drinks_ordered

Should not have more than fifteen children.

解决方案

It might seem like you could use a numeric id for the ordered drinks and then try something like this; it will fail since the ID is a string:

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

Instead, you can use a counter and validate the counter to be 1-15, and then validate the drink ID matches the counter.

"$customer_id": {
    "counter": {
       // counter can only be incremented by 1 each time, must be a number
       // and must be <= 15
       ".validate": "newData.isNumber() && newData.val() > 0 && newData.val() <= 15 && ((!data.exists() && newData.val() === 1) || (newData.val() === data.val()+1))"
    },
    "drinks_ordered": {
       // new record's ID must match the incremented counter
       "$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"
       }
    }
}

Naturally, your drinks will look something like this:

 /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

Now before a client could add another drink, they would have to set the counter to 4 (which is the only thing it can be set to) and then add the drink with that same ID.

A little roundabout, but it does do the job : )

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