Yii 中的更新查询

I have one requirement in Yii where I have to update one table based on some condition. And I have to update the column with new_val = previous_value + new_val. But the code is not working as expected.

我试过的代码是

$update = Yii::app()->db->createCommand()
->update('tbl_post', array('star'=>('star' + 1),'total'=>('total' + $ratingAjax)),
'id=:id',array(':id'=>$post_id));

在正常查询中，查询将是

UPDATE tbl_post set star= star + 1,total = total + '$ratingAjax' where id = 1

$update = Yii::app()->db->createCommand()
    ->update('tbl_post', 
        array(
            'star'=>new CDbExpression('star + 1'),
            'total'=>new CDbExpression('total + :ratingAjax', array(':ratingAjax'=>$ratingAjax))
        ),
        'id=:id',
        array(':id'=>$post_id)
    );