Firebase check for existing user in swift
<p>I have VC where by I am creating a new user and adding to firebase. The user inserts no problem into the database but I am having trouble checking if a user with the same name already exists. I have tried using various query statements within firebase but not getting any joy. Does anyone know how I can modify my code to accomplish this? here it is :-</p>

<pre><code> @IBAction func submitUsername(sender: AnyObject) {

    if let user = usernameTxtField.text where user != "" {

       let username = DataService.ds.REF_USERS.childByAppendingPath("username")

       username.observeSingleEventOfType(.Value, withBlock: {  snap in

            if let usernameDoesNotExist = snap.value as? NSNull {
              let currentUser = DataService.ds.REF_USER_CURRENT.childByAppendingPath("username")
              self.usernameTxtField.text = user
              currentUser.setValue(user)
            }
       })

       dismissViewControllerAnimated(true, completion: nil)

    } else {
        showAlert("Username Needed", msg: "please choose a username")
    } 
}
</code></pre>

<p>and the firebase data structure</p>

<pre><code>"users" : {
    "000591b6-ba88-4670-9cb7-edd79c273eab" : {
      "newuser" : "added to FB",
      "provider" : "password"
    },
    "05551c90-27fe-4969-b45d-279a72583857" : {
      "newuser" : "added to FB",
      "provider" : "password"
    },
    "0bbff1b0-681a-4fef-adc0-c4779a4b2100" : {
      "provider" : "password",
      "testing" : "emailtest",
      "username" : "richy446"
    },
    "12f35e7a-2430-49b3-a287-f7b1daf6a121" : {
      "newuser" : "added to FB",
      "provider" : "password",
      "username" : "richy445"
    },
    "15a45560-cc81-4c2d-bb59-b8ce6b125d68" : {
      "provider" : "password",
      "testing" : "emailtest",
      "username" : "richy445"
    },
    "1c135936-6dd0-4189-bcd7-e8036e9736f9" : {
      "newuser" : "added to FB",
      "provider" : "password"
    },
    "21591e29-f083-452c-b74f-cc567cecb183" : {
      "newuser" : "added to FB",
      "provider" : "password"
    },
    "8b385aa7-0e80-426b-a385-5b3317c894d0" : {
      "newuser" : "added to FB",
      "provider" : "password",
      "username" : "richy445"
    }
}
</code></pre>