using calculated value in custom sort

Question

I've been trying to apply this post:

How to add dynamic field in magento collection?

to my own simple scenario.

I have implemented a rewrite of Mage_Catalog_Model_Resource_Product_Collection to handle a custom sort option in catalog view.

I need to pull two attributes (msrp and special_price) do some math with them to create a 'virtual' field upon which to sort.

class NAMESPACE_MODULE_Model_Resource_Product_Collection
extends Mage_Catalog_Model_Resource_Product_Collection
{


public function addAttributeToSort($attribute, $dir = self::SORT_ORDER_ASC)
{
if ($attribute == 'rebate' && $storeId != 0) {
        $this->addPriceData();
        $this->addAttributeToFilter(
            array(
                array(
                    'attribute' => 'special_price_type',
                    'value'     => array('1340', '1339')
                )
            )
        );
        $this->addAttributeToSelect(
            array('special_price', 'msrp')
        );

        $this->addExpressionAttributeToSelect(
            'ratio',
            "[pseudocode: ( 1 - (mrsp - special_price))*100 ]"

        ); //select
        $this->getSelect()->order("ratio {$dir}");

        return $this;
    }

Struggling to understand how to manipulate those values in addExpressionAttributeToSelect. Nor am I clear whether those values are even exposed at this point. I would assume so because of:

   $this->addAttributeToSelect(
            array('special_price', 'msrp')
        );

However dropping a breakpoint there and stepping through the code, I can't find them anywhere in the instantiated object.

Any guidance welcome!

Answer 1

unrelated issue was causing interference. Working code below.

 if ($attribute == 'rebate' && $storeId != 0) {
        $this->addPriceData();
        $this->addAttributeToFilter(
            array(
                array(
                    'attribute' => 'special_price_type',
                    'value'     => array('1340', '1339')
                )
            )
        );
        $this->addAttributeToSelect(array(
            'msrp',
            'price',
            'special_price'
        ));
        $this->addExpressionAttributeToSelect(
            'ratio',
            '(ROUND( ( 1 - ( {{price}} - {{special_price}} ) )*100 ))',
            array(
                'special_price' => 'special_price',
                'price'         => 'price',
            )
        ); //
        $this->getSelect()->order("ratio {$dir}");
        return $this;
    }