Firebase database rules - some notes

In this below rules, newData seems to be something that is future once after the write is done. 

So, to check something on the newData, can place it in validate. 

{  

  "rules": {

    "myproj": {

      "chats":{

         "$roomId":{

            ".read":  "auth.uid != null",

            ".write":  "auth.uid != null",

            ".validate" : "newData.child('nick').val() == 'myproj11'"  

         }

      }

    }

  }

}