rules.txt

{
    "rules": {        
        "users": {
            "$user_id": {
                ".read": "auth != null &&
                    auth.uid == $user_id ||
                    root.child('users/' + auth.uid + '/roomId').val() == data.child('roomId').val()",
                ".write": "auth != null &&
                    auth.uid == $user_id ||
                    root.child('users/' + auth.uid + '/roomId').val() == data.child('roomId').val()"
            }
        },
        "rooms": {
            "$room_id": {
                ".read": "auth != null &&
                    data.child('/started').val() == false ||
                    data.child('/users/' + auth.uid).exists()",
                ".write": "auth != null &&
                    !data.exists() ||
                    data.child('started').val() == false ||
                    data.child('/users/' + auth.uid).exists()"
            }
        }
    }
}