在注册的时候怎么判断用户已经是否存在，并把错误信息显示出来？求解！ [ 1.1 版本 ]

控制器：
//登录

public function actionLogin()
{
    
    $this->layout="//layouts/column3";
    $model=new LoginForm;

    // if it is ajax validation request
    if(isset($_POST['ajax']) && $_POST['ajax']==='login-form')
    {
        echo CActiveForm::validate($model);
        Yii::app()->end();
    }

    // collect user input data
    if(isset($_POST['LoginForm']))
    {   

        $model->attributes=$_POST['LoginForm'];
        $model->password = md5($_POST['LoginForm']['password']);
        // validate user input and redirect to the previous page if valid
        if($model->validate() && $model->login()) {
            //var_dump(yii::app()->user->getState('userName'));exit();
            if((yii::app()->user->getState('isAdmin'))==true) {
                $this->redirect(array('meeting/index'));
            }
            $this->redirect(Yii::app()->user->returnUrl);
        }
    }
    // display the login form
    $this->render('login',array('model'=>$model));
}

//注册

public function actionCreate()
{

    $this->layout = '//layouts/column3';
    $model=new Person;

    
    // Uncomment the following line if AJAX validation is needed
    // $this->performAjaxValidation($model);

    if(isset($_POST['Person']))
    {
        $model->attributes=$_POST['Person'];
        $model->password = md5($_POST['Person']['password']);
        $model->repeat_password = md5($_POST['Person']['repeat_password']);
        $model->state = 1;
        if($model->save())
            $this->redirect(array('login'));
    }

    $this->render('register',array(
        'model'=>$model,
    ));
}