example of sorting collection by multiple columns

Observer.php

<?php

/**
 * Atwix
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to [email protected] so we can send you a copy immediately.
 * @category    Atwix Mod
 * @package     Atwix_Tweaks
 * @author      Atwix Core Team
 * @copyright   Copyright (c) 2014 Atwix (http://www.atwix.com)
 * @license     http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */
class Atwix_Tweaks_Model_Observer
{
    /**
     * apply sorting in tagged product listing
     * @param $event
     */
    public function catalogProductCollectionLoadBefore($event)
    {
        /** @var Mage_Tag_Model_Resource_Product_Collection $collection */
        $collection = $event->getCollection();
        if (get_class($collection) == 'Mage_Tag_Model_Resource_Product_Collection') {
            //adding news_from_date attribute to select doesn't work for some reason
            $collection->addAttributeToSelect(array('created_at'));
            $select = $collection->getSelect();
            if (!$collection->isEnabledFlat()) {
                $attributeModel = Mage::getModel('eav/entity_attribute')->loadByCode('catalog_product', 'news_from_date');
                $newsTableName = $attributeModel->getBackendTable();
                $select->joinLeft(
                    array('news_table' => $newsTableName),
                    new Zend_Db_Expr("e.entity_id = news_table.entity_id AND attribute_id = " . $attributeModel->getAttributeId() . " AND news_table.store_id = 0"),
                    array('news_table.value')
                );
                $select->columns(array('sorting_field' => new Zend_Db_Expr('IF(news_table.value IS NULL, e.created_at, news_table.value)')));
            } else {
                $select->columns(array('sorting_field' => new Zend_Db_Expr('IF(news_from_date IS NULL, e.created_at, news_from_date)')));
            }
            $select->order('sorting_field desc');
        }
    }
}