Overridding a Shipping Method - What am I missing

Question

I've written many many modules before but for some reason my shipping module won't override an exsiting Magneto shipping method. Is that allowed? What am I missing here? The module name shows up in the advanced tab of the configuration area, so it's getting loaded, but nothing is happening. Any hints?

Code

etc/modules/Ssi_Shipping.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Ssi_Shipping>
            <active>true</active>
            <codepool>local</codepool>
        </Ssi_Shipping>
    </modules>
</config>

local/Ssi/Shipping/etc.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Ssi_Shipping>
            <version>0.1.0</version>
        </Ssi_Shipping>
    </modules>
    <global>
        <models>
            <shipping>
                <rewrite>
                    <carrier_tablerate>Ssi_Shipping_Model_Carrier_Tablerate</carrier_tablerate>
                </rewrite>

            </shipping>
        </models>
    </global>
</config>

local/Ssi/Shipping/Model/Carrier/Tablerate.php

<?php
class Ssi_Shipping_Model_Carrier_Tablerate 
    extends Mage_Shipping_Model_Carrier_Tablerate {

        public function isActive()
        {
            Mage::log("here! Ssi_Shipping_Model_Carrier_Tablerate");

            // check to see if it's disabled
            if (parent::isActive() == false)
                return false;

            // check in the shopping cart
            foreach( Mage::getSingleton('checkout/session')->getQuote()->getItemsCollection() as $item ){
                if ($item->getDeliveryFlag() == "test")
                    return true;
            }

            // if nothing is found then disable this option.
            return false;

        }


    }

Answer 1

There is a way but it is not obvious and required me to browse the shipping module source:

If you look at Mage_Shipping_Model_Config, you will discover that the code used as parameter for Mage::getModel() is taken from the store configuration. This code is NOT the standard code like 'shipping/carrier_tablerate', so it does not help overriding as usual.

Now you have to find out first what this code is. For example I wanted to override the matrixrate carrier, so I tested it like that:

$carrierConfig = Mage::getStoreConfig('carriers/matrixrate')
var_dump($carrierConfig['model']);

Yes, you can put this code anywhere on the page temporary but it is useful to have a separate file for such things that can be run from the command line (starting with Mage::app() to initialize Magento)

In my case the code was matrixrate_shipping/carrier_matrixrate so I had to change my config.xml like that:

<global>
    <models>
        <matrixrate_shipping>
            <rewrite>
                <carrier_matrixrate>my_class_name</carrier_matrixrate>
            </rewrite>
        </matrixrate_shipping>
    </models>

instead of

<global>
    <models>
        <matrixrate>
            <rewrite>
                <carrier_matrixrate>my_class_name</carrier_matrixrate>
            </rewrite>
        </matrixrate>
    </models>

Good Luck!

Answer 2

Check first that the model is being overridden at all. Try this:

var_dump(get_class(Mage::getModel("shipping/carrier_tablerate")));