users {
  $uid {
    username: "username1",
    gender: "xyz"
    email: "xyz"
  }
},
usernames {
    "username1": $uid"
}

{
  "rules": {
    "users": {
      "$uid": {
        ".write": "auth !== null && auth.uid === $uid",
        ".read": "auth !== null && auth.uid === $uid",
        "username": {
          ".validate": "
            !root.child('usernames').child(newData.val()).exists() ||
            root.child('usernames').child(newData.val()).val() == $uid"
        }
      }
    },
    "usernames" : {
      ".write": "!data.exists() && auth!= null",
      ".validate": "newData.val() == auth.uid"  <---- I can't get this too work
    }
  }
}

"usernames" : {
  "$username": {
  ".write": "!data.exists() && auth!= null",
  ".validate": "newData.val() == auth.uid"
  }
 },
}