cakephp中的登录错误(或不存在）

我是CakePHP的初学者，遇到了一个问题，我不知道这是否是一个bug。我只能在注册用户后登录，如果我注销并尝试再次登录，它不起作用，它会说我的用户名或密码是错误的：（没错，t.t，我确信数据在数据库中。这很奇怪，因此我总是必须注册一个新用户才能访问我的系统。）。我的代码如下：

AppController:

class AppController extends Controller {
   public $components = array(
        'Session',
        'Auth' => array(
            'loginRedirect' => array('controller' => 'users', 'action' => 'lista_tudo'),
            'logoutRedirect' => array('controller' => 'users', 'action' => 'lista_tudo'),
            'authorize' => array('Controller')
        )
    );
    function beforeFilter() {
        $this->Auth->allow('login','add');
    }
    public function isAuthorized($user) {
        return true; // Admin pode acessar todas actions
}
}

用户控制器：

    public $uses = array('User','UserGroup','Group');
    public $name = "Users";
    public $helpers = array('Html','Form','Meu');
    public function beforeFilter() {
        parent::beforeFilter();
        $this->Auth->allow('logout','login','add');
    }
    public function login() {
        var_dump($this->Auth->loggedIn());
        if($this->request->is('post')){
            if ($this->Auth->login()) {
                $this->Session->setFlash('Logado com sucesso');
                $this->redirect($this->Auth->redirect());
            } else {
                $this->Session->setFlash('Usuário e/ou senha inválido');
            }
        }
    }
    public function logout() {
        $this->redirect($this->Auth->logout());
    }
}

登录视图

<div class="users form">
<?php echo $this->Session->flash('auth'); ?>
<?php echo $this->Form->create('User');?>
    <fieldset>
        <legend><?php echo __('Please enter your username and password'); ?></legend>
        <?php echo $this->Form->input('username');
        echo $this->Form->input('password');
    ?>
    </fieldset>
<?php echo $this->Form->end('Login');?>
<?php echo $this->Html->link('Logout',array('action'=>'logout')); ?>
</div>

用户型号：

public $name='User';
        public $hasMany = array(
                'UserGroup'=>array(
                        'className'=>'UserGroup'
                    )
            );
        public $validate = array(
            'username'=>array(
                    'rule'=>'isUnique',
                    'message'=>'Usuário já existe'
                )
        );
        public function beforeSave($options = array()) {
        if (isset($this->data[$this->alias]['password'])) {
        $this->data[$this->alias]['password'] = AuthComponent::password($this->data[$this->alias]['password']);
        }
        return true;
        }