Trim Function in Yii Model

From my sourcecode, I gave the information for trim function in yii framework. When we update the record, This function will apply to all fields.

<?php
class User extends CActiveRecord
{
  public static function model($className=__CLASS__)
  {
      return parent::model($className);
  }
  public function tableName()
  {
      return 'tbl_user';
  }
  public function rules()
  {                    array('username, password, firstname, 
                                  lastname, email, address    ', 
                             'filter', 'filter'=>'trim'),
  }
}
?>