在CakePHP框架上使用getJSON（）

I'm using AJAX to check whether a username is in the database or not. Here is the controller code:

public function check_username(){
    if($this->request->is('json')){
        $user = $this->User->find('count', array('conditions' => array('username' => $this->request->data['username'])));
        if($user === 0)
            echo '1';
        else
            echo false;
    }
    $this->autoRender = false;
}

And I used AJAX on the view. That was ok and now I'm trying to use Json and changed my controller code to:

public function check_username(){
    if($this->request->is('json')){
        $user = $this->User->find('count', array('conditions' => array('username' => $this->request->data['username'])));
        echo json_encode($user);
    }
    $this->autoRender = false;
}

This is my view code. This isn't working (no alert is being appeared):

var un = $('#username').val();
$.getJSON(
    '/oes/users/check_username',
    {username:un},
    function(result){
    alert(result);
});

Now anybody please help me make it working. I'm also new to Json. Thanks in advance.

public function check_username(){
    if($this->request->is('json')){
        $user = $this->User->find('count', array('conditions' => array('username' => $this->request->data['username'])));
        echo json_encode($user);
        exit;

    }
    $this->autoRender = false;
}

if it works , that the problem comes from loading view , (due to autoRender=false);