How to make login with only one field with CakePHP2

I am trying to make a login by getting/authorizing only one input user_number (Not username - password).

I made my current login page with the following way: Cakephp2.x simple login

Keep it simple

If you only have one way of identifying users, the simplest (and therefore recommended) way to identify users would be to define your own login function. e.g.:

public function login() {
    if ($this->request->is('post')) {
        $number = $this->request->data['User']['user_number'];
        $user = $this->User->findByUserNumber($number);
        if ($user && $this->Auth->login($user)) {
            return $this->redirect($this->Auth->redirectUrl());
        } else {
            $this->Session->setFlash(__('User %d doesn\'t exist', $number), 'default', array(), 'auth');
        }
    }
}

Note that this varies very little from the standard way of logging a user in with Cake 2.x