How i can get dependedntData of cache

Yii 1.1 General Discussion
1

When i try getDependentData - thay return null. But in query log i can find this query.


		// ---- get tree of comments ------

		$dependency = new CDbCacheDependency("SELECT COUNT(id) as cnt FROM `comments` WHERE `post_id` = {$post->id}  LIMIT 1");

		$comments   = Comment::model()->cache(Yii::app()->params['cache']['comments'], $dependency,2)->with('user')->getTree()->findAll('post_id = :pid', array(':pid' => $post->id));		

		$commentCount = $dependency->getDependentData();
2

why do you need to get a dependent data?