实例model的问题

请教，我用

//插入数据
    public function insert()
    {
        $Dome = M("Demo","CommonModel");
        //$Dome = CommonModel("Demo");
        //$Dome->set_echo("模块输出");
        $Dome->Create();
        $list = $Dome->add();
        $this->redirect("index");
    }



系统报Fatal error: Class 'CommonModel' not found in D:\web\PHP\PHPDome\Runtime\~runtime.php on line 2
$Dome = M("Demo","CommonModel");是手册上提供的实例方法
请问这个错误的原因谢谢！！！