yii save model return id null

    /**
     * Creates a new model.
     * If creation is successful, the browser will be redirected to the ‘view‘ page.
     */
    public function actionCreate()
    {
        $news=new News;
        $news_item = new NewsItem;
        $news->item = $news_item;
        // Uncomment the following line if AJAX validation is needed
        // $this->performAjaxValidation($news);
        if (isset($_POST[‘News‘])) {
            $news->attributes=$_POST[‘News‘];
            if ($news->save()) {
                $_POST[‘NewsItem‘][‘header_id‘] = $news->id; 
                $news_item->attributes=$_POST[‘NewsItem‘];
                if ($news_item->save()) {
                    $this->redirect(array(‘view‘,‘id‘=>$news->id));
                }
                else {
                    $news->deleteByPk($news->id);
                    $this->redirect(array(‘admin‘));
                }
            }
        }
        $this->render(‘create‘,array(
            ‘model‘=>$news,
        ));
    }