Implementing an event observer in Magento - events

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:
You wish to reduce the stock level of all associated products of the parent just purchased
You wish to reduce by only 1
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()
;
}
}
}
}
}

Answer 2

There are several possibilities with simplest being just checkout_type_onepage_save_order_after
however do you also need to revert the stock if order get's cancelled or execute this only when payment is made or items are shipped?