public function __construct($sql=null)
{
    $this->sql=$sql;
}

public function __sleep()
{
    $this->_db=null;
    return array_keys((array)$this);
}

protected function generateDependentData()
{
    if($this->sql!==null)
    {
        $db=$this->getDbConnection();
        $command=$db->createCommand($this->sql);
        if(is_array($this->params))
        {
            foreach($this->params as $name=>$value)
                $command->bindValue($name,$value);
        }
        if($db->queryCachingDuration>0)
        {
            // temporarily disable and re-enable query caching
            $duration=$db->queryCachingDuration;
            $db->queryCachingDuration=0;
            $result=$command->queryRow();
            $db->queryCachingDuration=$duration;
        }
        else
            $result=$command->queryRow();
        return $result;
    }
    else
        throw new CException(Yii::t('yii','CDbCacheDependency.sql cannot be empty.'));
}

protected function getDbConnection()
{
    if($this->_db!==null)
        return $this->_db;
    else
    {
        if(($this->_db=Yii::app()->getComponent($this->connectionID)) instanceof CDbConnection)
            return $this->_db;
        else
            throw new CException(Yii::t('yii','CDbCacheDependency.connectionID "{id}" is invalid. Please make sure it refers to the ID of a CDbConnection application component.',
                array('{id}'=>$this->connectionID)));
    }
}