Implementing an event observer in Magento

Question

Im starting to understand how Magento Event Observers work and hoping someone could help me figure out what Event I would need to seek out to get my potential module to work. I have certain products that have an attribute called "child skus" which is an array of SKU numbers of other products in my inventory.

Example:

Name: Totally Awesome Product

SKU: TP-1212

Child SKUS: GP-3232,FD-4332,VC-5332

Every time someone purchases TP-1212 I also need Magento to deduct that quantity from those child skus behind the scenes. Does anyone know if there is an event dispatcher for after a purchase has been completed that would handle something like that??

Answer 1

This is a little tricky and there are most likely some edge cases not covered in the below code - Also the answer assumes the following:

1. You wish to reduce the stock level of all associated products of the parent just purchased
2. You wish to reduce by only 1
3. There are no further complications or other conditions that must be met/dealt with

---

Code:

So, you obviously need to create a module first. The config.xml needs to declare the observer which will listen to checkout_type_onepage_save_order_after. (Note: there are other events you can listen to in order to achieve your goal).

The config.xml will contain the following code at a minimum:

<?xml version="1.0"?>
<config>
    <modules>
        <YourCmpany_YourModule>
            <version>1.0.0</version>
        </YourCmpany_YourModule>
    </modules>
    <frontend>
        <events>
            <checkout_type_onepage_save_order_after>
                <observers>
                    <yourmodule_save_order_observer>
                        <class>YourCompany_YourModule_Model_Observer</class>
                        <method>checkout_type_onepage_save_order_after</method>
                    </yourmodule_save_order_observer>
                </observers>
            </checkout_type_onepage_save_order_after>
        </events>
    </frontend>
    <global>
        <models>
            <yourmodule>
                <class>YourCompany_YourModule_Model</class>
            </yourmodule>
        </models>
    </global>
</config>

Then in your observer:

<?php

class YourCompany_YourModule_Model_Observer
{
    public function checkout_type_onepage_save_order_after(Varien_Event_Observer $observer)
    {
        $order = $observer->getEvent()->getOrder();

        /**
         * Grab all product ids from the order
         */
            $productIds = array();
        foreach ($order->getItemsCollection() as $item) {
            $productIds[] = $item->getProductId();
        }

        foreach ($productIds as $productId) {

            $product = Mage::getModel('catalog/product')
                ->setStoreId(Mage::app()->getStore()->getId())
                ->load($productId);

            if (! $product->isConfigurable()) {
                continue;
            }

            /**
             * Grab all of the associated simple products
             */
            $childProducts = Mage::getModel('catalog/product_type_configurable')
                        ->getUsedProducts(null, $product);

            foreach($childProducts as $childProduct) {

                /**
                 * in_array check below, makes sure to exclude the simple product actually 
                 * being sold as we dont want its stock level decreased twice :)
                 */
                if (! in_array($childProduct->getId(), $productIds)) {

                    /**
                     * Finally, load up the stock item and decrease its qty by 1
                     */
                    $stockItem = Mage::getModel('cataloginventory/stock_item')
                                    ->loadByProduct($childProduct)
                                    ->subtractQty(1)
                                    ->save()
                                ;
                }
            }
        }
    }
}