Cakephp Auth problem : Undefined property: User::$alias - cakephp

Question

I keep getting this error over a day right now.
I search over and over , but i couldnt get an answer.
Probably i am making sometihing stupid.
Here is my enviroment;
UserController :
class UsersController extends AppController {
var $name = 'Users';
var $components = array('Email');
function login() {
}
function home() {
pre('login oldunuz');
}
function logout() {
$this->Session->setFlash(__('Goodbye!', true));
$this->redirect($this->Auth->logout());
}
....
}
AppController :
class AppController extends Controller {
var $components = array('Session','Auth');
function beforeFilter() {
$this->Auth->loginError = 'You didnt supply valid identification.';
$this->Auth->authError = 'You are not validated to see this location.';
$this->Auth->allowedActions = array('register','confirm');
$this->Auth->userScope = array('User.active' => 1);
}
function beforeRender() {
$this->set('User', $this->Session->read('Auth.User'));
}
}
UserModel:
class User extends AppModel {
var $name = 'User';
var $displayField = 'username';
var $validate = array(
'username' => array(
'notempty' => array(
'rule' => array('notempty'),
'message' => 'Username field can not be empty..',
),
'u_unique' => array(
'rule'=>array('isUnique'),
'message' => 'The username you selected, is being used please choose another one.'
)
),
'email' => array(
'email' => array(
'rule' => array('email'),
'message' => 'Please provide a valid email.',
),
'e_unique' => array(
'rule' => array('isUnique'),
'message' => 'The email you selected, is being used please choose another one.'
)
),
----
/* if you want i send more of this code */
}
I dont how to fix this,
But for helping, cakephp says :
The error portion is :
if ($key == null)
{
$model =& $this->getModel();
return array($model->alias => $this->Session->read($this->sessionKey));
}
if i make $model->alias = "User" then it works but where ever another $model->alias is used, it explodes same as before,
I hope you have an answer, thanks all,