用于数据验证的Firebase规则

Question

Assuming a data structure like below, I want a rule that disallows adding new car makes, for example users cannot add "toyota". So, I need to .validate such that the .update must match an existing make.

I have tried using $cars with ".validate": "newData.val().contains($cars)" and all kind of other ways with no luck. Any help is appreciated.

My code is:

function setModel(model,make,key){
    if (model && make && key){
        var carsRef = ref.child('cars');
        var makeRef = carsRef.child(make);
        var modelRef = makeRef.child(model);
        var obj = {};
        obj[key] = true;           
        modelRef.update(obj, onComplete);
    }
}

The firebase looks like:

{
"cars" : {
   "chevrolet" : {
      "silverado" : {
         "id192874623" : true,
         "id786766663" : true
      }
   },
   "ford" : {
      "taurus" : {
         "id736273627" : true
      }
   },
   "honda" : {
      "accord" : {
         "id635263535" : true
      }
   }
}}

Answer 1

To disallow adding new car brands:

"cars": {
  "$brand": {
    ".validate": "data.exists()"
  }
}

This is covered in the section on existing data vs new data of the documentation.