Detect inventory change

Question

I need to detect whenever the inventory level of a product changes. I've had some success using the cataloginventory_stock_item_save_after event which is triggered when inventory is changed in the backend, or when an order is cancelled on the frontend (via Paypal), but is not triggered when a product is purchased from the frontend.

I'm hooking into the cataloginventory_stock_item_save_after event like this:

<global>
    <events>
        <cataloginventory_stock_item_save_after>
            <observers>
                <cataloginventory_stock_item_save_after_handler>
                    <type>model</type>
                    <class>stockchange/observer</class>
                    <method>stockChange</method>
                </cataloginventory_stock_item_save_after_handler>
            </observers>
        </cataloginventory_stock_item_save_after>
    </events>

<?php
class FashionBunker_StockChange_Model_Observer {
    public function stockChange(Varien_Event_Observer $observer) {

Do I need to use another event to capture the inventory change when a customer purchases something, or is there something wrong with the way I've hooked into the event?

Answer 1

It has been some time ago that I build something for this, I had to listen to multiple observers because they where not all handled by the save of the cataloginventory, I have to following code:

    <events>
        <cataloginventory_stock_item_save_commit_after>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>catalogInventorySave</method>
                </genmato_stockupdate>
            </observers>
        </cataloginventory_stock_item_save_commit_after>
        <sales_model_service_quote_submit_before>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>subtractQuoteInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_model_service_quote_submit_before>
        <sales_model_service_quote_submit_failure>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>revertQuoteInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_model_service_quote_submit_failure>
        <sales_order_item_cancel>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>cancelOrderItem</method>
                </genmato_stockupdate>
            </observers>
        </sales_order_item_cancel>
        <sales_order_creditmemo_save_after>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>refundOrderInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_order_creditmemo_save_after>
    </events>

And in the observer the following code:

public function catalogInventorySave(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $event = $observer->getEvent();
        $_item = $event->getItem();

        if ((int)$_item->getData('qty') != (int)$_item->getOrigData('qty')) {
            $params = array();
            $params['product_id'] = $_item->getProductId();
            $params['qty'] = $_item->getQty();
            $params['qty_change'] = $_item->getQty() - $_item->getOrigData('qty');
        }
    }
}

public function subtractQuoteInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $quote = $observer->getEvent()->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getTotalQty() * -1);
        }
    }
}

public function revertQuoteInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $quote = $observer->getEvent()->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getTotalQty());
        }
    }
}

public function cancelOrderItem(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $item = $observer->getEvent()->getItem();
        $qty = $item->getQtyOrdered() - max($item->getQtyShipped(), $item->getQtyInvoiced()) - $item->getQtyCanceled();
        $params = array();
        $params['product_id'] = $item->getProductId();
        $params['sku'] = $item->getSku();
        $params['qty'] = $item->getProduct()->getStockItem()->getQty();
        $params['qty_change'] = $qty;
    }
}

public function refundOrderInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $creditmemo = $observer->getEvent()->getCreditmemo();
        foreach ($creditmemo->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getQty());
       }
    }
}

Hope this is a bit what you are looking for.

Answer 2

You cannot use any event related to the stock item model, because Magento uses an optimized SQL query to decrease stock for all ordered items at once, bypassing the model.

I solved this with a rewrite of Mage_CatalogInventory_Model_Stock where I added an additional event:

<?php
/**
 * Add events to observe stock qty change
 * 
 * @author Fabian Schmengler
 *
 */
class SGH_ShippingExpress_Model_CatalogInventory_Stock
    extends Mage_CatalogInventory_Model_Stock
{
    const EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE = 'cataloginventory_stock_item_correct_qty_before';
    const EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER = 'cataloginventory_stock_item_correct_qty_after';

    /**
     * (non-PHPdoc)
     * @see Mage_CatalogInventory_Model_Stock::registerProductsSale()
     */
    public function registerProductsSale($items)
    {
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
            'stock'     => $this,
            'items_obj' => (object)array('items' => &$items),
            'operator'  => '-'
        ));
        $result = parent::registerProductsSale($items);
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER, array(
            'stock'          => $this,
            'items'          => $items,
            'fullsave_items' => $result,
            'operator'       => '-'
        ));
        return $result;
    }
    /**
     * (non-PHPdoc)
     * @see Mage_CatalogInventory_Model_Stock::revertProductsSale()
     */
    public function revertProductsSale($items)
    {
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
            'stock'     => $this,
            'items_obj' => (object)array('items' => &$items),
            'operator'  => '+'
        ));
        $result = parent::revertProductsSale($items);
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER, array(
            'stock'          => $this,
            'items'          => $items,
            'fullsave_items' => $result,
            'operator'       => '+'
        ));
        return $result;
    }
}

Then the observer for cataloginventory_stock_item_correct_qty_after can look like this:

    /**
     * @var $items array array($productId => array('qty'=>$qty, 'item'=>$stockItem))
     */
    $items = $observer->getItems();
    foreach ($items as $productId => $item) {
        $stockItem = $item['item'];
        $product = $stockItem->getProduct();

        // Do anything you need with $stockItem and $product here

    }

I recommend not to do heavy processing or additional database calls (which are necessary to detect if the product is out of stock for example), but to add the products to a queue that is processed by a cronjob, to minimize additional load time for the user.