Which Attribute Type for layered nav and filterable numeric value

Question

I would like to put an Attribute called "Volume" to my Magento Online Shop. Products should be sorted by this attribute and it should be filterable through the layered navigation. since text attributes are not filterable in layered navigation and multi select fields not sortable i actually dont know how to acomplish that. any idea maybe ? i was thinking about a price attribute but then it would be there in EUROs not in Liters for example...

Answer 1

You can override the order in an observer and provide SQL that takes numeric values into account. This is some kind of a workaround since Magento doesn't allow for elegant sort order modification.

config.xml

Observer definition listening to event catalog_product_collection_load_before – i.e. last one before query gets rendered:

<config>
    <frontend>
        <events>
            <catalog_product_collection_load_before>
                <observers>
                    <namespace_catalog_product_collection_load_before>
                        <type>singleton</type>
                        <class>namespace_module/observer</class>
                        <method>setSortOrder</method>
                    </namespace_catalog_product_collection_load_before>
                </observers>
            </catalog_product_collection_load_before>

Observer.php

public function setSortOrder (Varien_Event_Observer $observer)
{
    $numericallySortedAttributeCode = 'test_attribute';

    $request = Mage::app()->getRequest();
    $order = $request->getParam ( 'order' );

    if ( $order == $numericallySortedAttributeCode )
    {
        $dir = strtoupper ( $request->getParam ( 'dir' ) );
        $dir = in_array ( $dir, array ( Varien_Db_Select::SQL_ASC, Varien_Db_Select::SQL_DESC ) ) ? $dir : 'DESC' ;

        $observer
            ->getCollection()
            ->getSelect()
            ->reset ( Zend_Db_Select::ORDER )
            ->order ( new Zend_Db_Expr ( 'CAST(`' . $numericallySortedAttributeCode . '` AS SIGNED) ' . $dir ) )
        ;
    }
}

This is tested with attribute values of type integer. If you use alphanumeric values like '30 fl.oz.', you might have to modify the SQL CAST. You probably also wanna edit the attribute code and the condition of when to apply the sort modification – above code obviously only works if the sort order in question is sent explicitly as a request parameter aka not used implicitly as default sort order.