title: get count in relation table in yii2 Activerecord
body: <p>I have two table for post and user. I want to show post count of user in users list gridview. In yii 1 I use this in model to define a relation for this purpose:</p>
  
  <pre><code>'postCount' =&gt; array(self::STAT, 'Post', 'author',
              'condition' =&gt; 'status = ' . Post::ACTIVE),
  
  ...
  User:find...().with('postCount').....
  </code></pre>
  
  <p>But i don't know how to implement this in Yii2 to get count of post in User:find():with('...') to show in gridview.<br>
  Anyone try this in yii2?</p>