CDbMessageSource

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

protected function loadMessages($category,$language)
{
    if($this->cachingDuration>0 && $this->cacheID!==false && ($cache=Yii::app()->getComponent($this->cacheID))!==null)
    {
        $key=self::CACHE_KEY_PREFIX.'.messages.'.$category.'.'.$language;
        if(($data=$cache->get($key))!==false)
            return unserialize($data);
    }

    $messages=$this->loadMessagesFromDb($category,$language);

    if(isset($cache))
        $cache->set($key,serialize($messages),$this->cachingDuration);

    return $messages;
}

protected function loadMessagesFromDb($category,$language)
{
    $command=$this->getDbConnection()->createCommand()
        ->select("t1.message AS message, t2.translation AS translation")
        ->from(array("{$this->sourceMessageTable} t1","{$this->translatedMessageTable} t2"))
        ->where('t1.id=t2.id AND t1.category=:category AND t2.language=:language',array(':category'=>$category,':language'=>$language))
    ;
    $messages=array();
    foreach($command->queryAll() as $row)
        $messages[$row['message']]=$row['translation'];

    return $messages;
}