title: Yii cdbcriteria select a relation's columns
body: <p>I am having very difficult time to select usernames of all posts in the blog demo given in Yii..</p>
  
  <p>author is relation of post class with user...</p>
  
  <pre><code>$criteria = new CDbCriteria;
  $criteria-&gt;with='author';
  $criteria-&gt;select='author.username';
  $dataProvider=new CActiveDataProvider('Post', array(
      'criteria' =&gt; $criteria,
  ));
  var_dump($dataProvider-&gt;getData());
  </code></pre>
  
  <p><strong>Error:</strong></p>
  
  <p>Active record "Post" is trying to select an invalid column "author.username". Note, the column must exist in the table or be an expression with alias.</p>