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,
		));
	}

原来不是别的bug，因为 数据库中没有设置 id主键为 autoincrement