gistfile1.js

{
  "chat": {
     // the list of chats may not be listed (no .read permissions here)
     
     // a chat conversation
     "$key": {
         
         // if the chat hasn't been created yet, we allow read so there is a way 
         // to check this and create it; if it already exists, then authenticated 
         // user (specified by auth.id) must be in $key/users
        ".read": "auth != null && (!data.exists() || data.child('users').hasChild(auth.id))",
        
        // list of users authorized to participate in chat
        "users": {
           // if the list doesn't exist, anybody can create it
           // if it already exists, only users already in the list may modify it
           ".write": "!data.exists() || data.hasChild(auth.id)",
           "$acc": {
              // for now the value is just a 1, later it could be a read/write/super privilege
              ".validate": "newData.isNumber()"
           }
        },
        
        // timestamps recording last time each user has read this chat
        "last": {
           "$acc": {
              // may only written by the authenticated user and if user is in $key/users
              ".write": "$acc === auth.id && root.child('chat/'+$key+'/users').hasChild($acc)",
              ".validate": "newData.isNumber()"
           }
        },
        
        "messages": {
           "$msg": {
              // to write a message, it must have all three fields (usr, ts, and msg)
              // and the person writing must be in $key/users
              ".write": "root.child('chat/'+$key+'/users').hasChild(auth.id)",
              ".validate":"newData.hasChildren(['ts', 'usr', 'msg'])",
              "usr": {
                 // may only create messages from myself
                 ".validate": "newData.val() === auth.id"
              },
              "msg": {
                 ".validate": "newData.isString()"
              },
              "ts": {
                 ".validate": "newData.isNumber()"
              }
           }
        }
     }
  }
 
}