Kohana 3.3 更新期间的验证

我正在使用Kohana 3.3，并希望在我的控制器中验证用户输入，但它返回以下错误：

错误异常 [ 警告 ]：call_user_func_array（） 期望参数 1 是有效的回调，第二个数组成员不是有效的方法

这是我的控制器：

$this->template->user =Auth::instance()->get_user();
                $courseModel = Model::factory('courses');
                $object = Validation::factory($this->request->post());
                // $object->bind(':model', $courseModel);
                $object
                    ->rule('code', 'not_empty')
                    ->rule('code', 'Model_Courses::unique_code')
                    ->rule('code', array('max_length', array(':value', 32)))
                    ->rule('description', 'not_empty');
                if($object->check()) { //this is where the error triggers
                    $user = ORM::factory('courses', $this->request->param('id'))
                        ->values($_POST, array(
                            'code', 
                            'description', 
                        ));
                    $query = DB::update('courses')
                        ->set(array(
                            'code' => $_POST['code'], 
                            'description' => $_POST['description'],
                            ))
                        ->where('id', '=', $this->request->param('id'));
                    $result = $query->execute();
                    // Reset values so form is not sticky
                    $_POST = array();
                    $courses = ORM::factory('courses')
                        ->find_all();
                    $json = array();
                    foreach ($courses as $course) {
                        if($course->id != 1) $json[] = $course->as_array();
                    }
                    $data = json_encode($json);
                    // Display users table
                    $courseView = View::factory('courses/list');
                    $courseView->bind('content', $data);
                    $this->template->content = $courseView;

我的Model_Courses代码如下：

class Model_Courses extends ORM {
protected $_table_name = 'courses';
protected $_primary_key = 'id';
public function rules() {
    return array(
        'code' => array(
            array('not_empty'),
            array('max_length', array(':value', 32)),
            array(array($this, 'unique'), array(':field', ':value')),
        ),
        'description' => array(
            array('not_empty'),
        ),
    );
}
public static function unique_code($code)
{
    return ! DB::select(array(DB::expr('COUNT(code)'), 'total'))
        ->from('courses')
        ->where('code', '=', $code)
        ->execute()
        ->get('total');
}

}

我错过了什么？我遵循了此处的文档：

http://kohanaframework.org/3.3/guide/kohana/security/validation

请帮忙！