Customizing Magento by overwriting core-classes

There are several possibilities of overwriting single magento-methods or whole classes. Of course you could be just altering the core-classes themselves in a quick and dirty approach. however, you should avoid this practice under all circumstances, because it corrupts the update-facilities of your eCommerce-site. Once you are upgrading your magento-copy to a newer version, all your changes would get lost.

Luckily, overwriting native magento-classes is pretty straight-forward, and therefore there is no reason not to do it the right way.

In this example, we are overwriting the core method of the block-class “Onepage_Billing” of the Mage_Checkout-core module to add another default address. This is just an example to show how you actually add new functionality by inheriting from a Magento-coreclass and overwriting particular methods.

Extend Magento Core-Class

app/code/local/Test/Checkout/Block/Onepage/Billing.php

First we derive from the class that we actually want to overrule. In our example, we are simply overwriting the core Onepage_Billing-class of the Mage_Checkout-core-module with an inherited class called

Test_Checkout_Block_Onepage_Billing

, which declares and therfore overrules the method

getAddressesHtmlSelect

. In this method we can add and/or alter all the functionality that we actually need.

<?php
class Test_Checkout_Block_Onepage_Billing extends Mage_Checkout_Block_Onepage_Billing
{
    public function getAddressesHtmlSelect($type)
    {
        if ($this->isCustomerLoggedIn()) {
            $options = array();
            foreach ($this->getCustomer()->getAddresses() as $address) {
                $options[] = array(
                    'value' => $address->getId(),
                    'label' => $address->format('oneline')
                );
            }

            $addressId = $this->getAddress()->getCustomerAddressId();
            if (empty($addressId)) {
                if ($type=='billing') {
                    $address = $this->getCustomer()->getPrimaryBillingAddress();
                } else {
                    $address = $this->getCustomer()->getPrimaryShippingAddress();
                }
                if ($address) {
                    $addressId = $address->getId();
                }
            }

            $select = $this->getLayout()->createBlock('core/html_select')
                ->setName($type.'_address_id')
                ->setId($type.'-address-select')
                ->setClass('address-select')
                ->setExtraParams('onchange="'.$type.'.newAddress(!this.value)"')
                ->setValue($addressId)
                ->setOptions($options);

            return $select->getHtml();
        }
        return '';
    }
}

Activate Module

app/etc/modules/Test_All.xml

Now it’s time to tell Magento that there is a module (in our example we have named it “Test”) which adds customized functionality to the Magento-core-functionality.

<?xml version="1.0"?>
<config>
    <!-- Please add items in alphabetic order. -->
    <modules>
        <Test_Checkout>
            <active>true</active>
            <codePool>local</codePool>
        </Test_Checkout>
     </modules>
</config>

Actual Overwrite

app/code/local/Test/Checkout/etc/config.xml

Now this is where the Magento-rewrite-magic pops in. By providing a rewrite-directive in the Test_Checkout-config.xml, Magento’s default behaviour can be overruled for certain pages, and we can make sure that our derived class is being loaded instead of the core Checkout-module from Magento.

<?xml version="1.0"?>
<config>
    <modules>
         <Test_Checkout>
            <version>0.1.0</version>
        </Test_Checkout>
    </modules>
    <global>
	    <blocks>
		    <checkout>
		        <rewrite>
		            <onepage_billing>Test_Checkout_Block_Onepage_Billing</onepage_billing>
		        </rewrite>
		    </checkout>
		</blocks>
   </global>
</config>

Clear Cache

Whenever we are changing Magento-settings by adding or modifying the underlying xml-configuration-files, we have to empty the cache (residing at app/var/cache) in order to have the changes come into effect. So before going wild because your changes don’t show, rather empty Magento’s file-cache.

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