Performance: Add Inventory Stock Levels on product listings list.phtml of all product types

Question

TL;DR, The requirement is having a stock level of inventory be displayed on the category product listing page with as little additional queries/memory with performance in mind that adheres to Magento's framework.

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After reading Vinai Kopp's article on preloading for scalability.

What is the best way to include the inventory stock levels on the category product listing pages (list.phtml) with as few additional queries/loads for performance sakes?

I am aware of a few approaches:

afterLoad() seems to work well with media_gallery inclusion without additional queries, however I've not been successful implementing the same approach with inventory.

$attributes = $_product->getTypeInstance(true)->getSetAttributes($_product);
$media_gallery = $attributes['media_gallery'];
$backend = $media_gallery->getBackend();
$backend->afterLoad($_product);

Direct SQL to gather the required data in parallel to the collection with a product_id key for instance. But looking for more of a means through the framework.

Currently I am simply loading the stock_item object via:

$_product->load('stock_item')->getTotalQty(); Which works, but I notice the addition of more queries to get the inventory stock totals of all products in the collection.

...

__EAV_LOAD_MODEL__ (Mage_Catalog_Model_Product, id: stock_item, attributes: NULL)
__EAV_LOAD_MODEL__ (Mage_Catalog_Model_Product, id: stock_item, attributes: NULL)
__EAV_LOAD_MODEL__ (Mage_Catalog_Model_Product, id: stock_item, attributes: NULL)

...

strangely, this works. The magic happens in Mage_Eav_Model_Entity_Abstract->load($object, $entityId, $attributes). If $attributes is empty it will call loadAllAttribute($object). So $product->load('blah') will load all missing attributes, including 'media_gallery' – William Tran Nov 19 '14 at 4:45

• https://stackoverflow.com/questions/7890616/get-product-media-gallery-images-from-a-product-collection-in-magento#comment42546135_23804663

Add needed values to the already loaded collection.

The obvious simple approach of adding the needed data to the top level production collection in the layer/filter, would seem to be the best approach.

I did notice an observer addInventoryDataToCollection() in Mage_CatalogInventory_Model_Observer that sounds like it would achieve such, but adding the method to a custom modules observer doesn't seem to be compatible.

<events>
    <catalog_product_collection_load_after>
        <observers>
            <inventory>
                <class>cataloginventory/observer</class>
                <method>addInventoryDataToCollection</method>
            </inventory>
        </observers>
    </catalog_product_collection_load_after>
</events>

Which results in:

Warning: Invalid argument supplied for foreach() in /app/code/core/Mage/CatalogInventory/Model/Resource/Stock/Item/Collection.php on line 71

Answer 1

The real issue here isn't preloading, it's accuracy. It's relatively easy to obtain the stock amount for a collection of products:

$products = Mage::getModel('catalog/product')->getCollection()
    ->addCategoryFilter($_category);
$stockCollection = Mage::getModel('cataloginventory/stock_item')
    ->getCollection()
    ->addProductsFilter($products);

Now with two queries, you have all the info you need. They're just hard to relate to one another, which can be fixed by using an associative array 'product_id' => 'stock' and writing a getter. Also, addProductsFilter can be optimized:

public function addProductIdsFilter(array $productIds)
{
    if(empty($productIds) {
        $this->_setIsLoaded(true);
    }
    $this->addFieldToFilter('main_table.product_id', array('in' => $productIds));
    return $this;
}

This saves you the type check and array cloning.

The problem now is Block HTML cache. This category page needs to be purged when any stock is updated on a product contained in it. As far as I know, this isn't standard, since only a stock status change purges a category page containing a product (or more accurately, a visibility change). So you'll need to observe at least cataloginventory_stock_item_before_save and maybe a few others and purge block html cache (and FPC cache) for that category page.

Answer 2

I see you've already accepted and no-doubt implemented something by now, however I'd like to point out how close you were with addInventoryDataToCollection() but it looks like you misquoted the config file or we're using vastly different versions of magento. My copy of CatalogInventory/etc/config.xml has the a different method called for catalog_product_collection_load_after

 <catalog_product_collection_load_after>
    <observers>
        <inventory>
            <class>cataloginventory/observer</class>
            <method>addStockStatusToCollection</method>
        </inventory>
    </observers>
 </catalog_product_collection_load_after>

addInventoryDataToCollection() is called in <sales_quote_item_collection_products_after_load>

The source for addStockStatusToCollection() is:

public function addStockStatusToCollection($observer)
{
    $productCollection = $observer->getEvent()->getCollection();
    if ($productCollection->hasFlag('require_stock_items')) {
        Mage::getModel('cataloginventory/stock')->addItemsToProducts($productCollection);
    } else {
        Mage::getModel('cataloginventory/stock_status')->addStockStatusToProducts($productCollection);
    }
    return $this;
}

You could either set the flag require_stock_items on the collection before it's loaded, probably not that easy for the block behind the category list, or you could call Mage::getModel('cataloginventory/stock')->addItemsToProducts($productCollection) manually on the collection, after it's already loaded. addItemsToProducts() gets all the StockItems for you and attaches them to your ProductCollection

public function addItemsToProducts($productCollection)
{
    $items = $this->getItemCollection()
        ->addProductsFilter($productCollection)
        ->joinStockStatus($productCollection->getStoreId())
        ->load();
    $stockItems = array();
    foreach ($items as $item) {
        $stockItems[$item->getProductId()] = $item;
    }
    foreach ($productCollection as $product) {
        if (isset($stockItems[$product->getId()])) {
            $stockItems[$product->getId()]->assignProduct($product);
        }
    }
    return $this;
}

Answer 3

Are you using Varnish or FPC at all, or plan to in the future?

We found with the amount of hole punches/ESI requests required on the product listings it almost wasn't worth having the caching in place so opted for a different approach.

We implemented a solution on one website that uses an AJAX request to a custom controller to retrieve the stock data for the products and the javascript handles the DOM updates. The additional request for the stock data takes ~100ms which doesn't impact on the overall (visible) page load time at all. Couple that with primed FPC dropping the page request down to under 100ms you have one rapid site with low performance overhead for displaying stock data on the product listings.

All you need to do template wise is add the productId to each products wrapper html so your javascript knows which stock data to apply to each product.

If you look at additional caching techniques for the actual stock data you could drop that well below 100ms by not having to init Mage/hit the database on each request.

Sorry if this isn't along the lines of what you're looking for but we found it to be the best approach for scalability and performance against our requirements.