How to set null value product in last in sorting on product listing page?

Question

Hello i have main 4 product like

     product_name  attribute_name attribute_value
----------------------------------------------------------
     test1           catname        abc
     test2           catname        ccc
     test3           catname        ###
     test4           catname        bbb

when i sort product bye catname on product listing page than i will get null value product test3 first , but i need it in last. i have to igonre null attribute value.

I have try in toolbar.phtml for asc and desc but its not which i want.

Can anyone help me??

Answer 1

You will need to rewrite the method Mage_Catalog_Model_Resource_Product_Collection::addAttributeToSort() and handle your attribute code separately.
Here is a similar example for showing products that have images on top of the ones that don't.
You can do a similar thing for your custom attribute.
Follow the example above, but make your new class look like this:

<?php
class Easylife_Sorting_Model_Resource_Product_Collection
    extends Mage_Catalog_Model_Resource_Product_Collection {
    public function addAttributeToSort($attribute, $dir = self::SORT_ORDER_ASC){
        //don't screw up the admin sorting
        if (Mage::app()->getStore()->getId() == Mage_Core_Model_App::ADMIN_STORE_ID){
            return parent::addAttributeToSort($attribute, $dir);
        }

        if ($attribute == 'catname') {
            $this->addAttributeToSelect($attribute);
            //add a 1|0 flag for products depending on:
            // - 0 - if the attribute value is null or empty
            // - 1 - if the attribute is not null
            $this->addExpressionAttributeToSelect(
                'has_catname',
                "(CASE WHEN {{catname}} = '' OR {{catname}} IS NULL THEN 0 ELSE 1 END)",
                array('catname'=>$attribute)
            );
            //sort by the flag set above 
            $this->getSelect()->order(array('has_catname DESC', 'catname '.$dir));
            return $this;
        }
        //if not sorting by catname let magento do its magic
        else {
            return parent::addAttributeToSort($attribute, $dir);
        }
    }
}

I haven't tested this but it should work...I think.