Magento 2 – Real Usage of Product Attribute: quantity_and_stock_status

I just tried your exact code (with save enabled) on my Magento installation and it just works without modification.

There has to be another issue at your machine when it gives a 404 - maybe the product is disabled automatically (i.e. getting status attribute 0) as it was out of stock?

In addition, I refactored your code to avoid usages of array access ['controller_action'], addData and getData. It is best practice to use the magic methods instead.

Moreover, using the event catalog_controller_product_init passes a loaded product instance in its parameters so you don't have to do the loading yourself.

<events>
    <catalog_controller_product_init>
        <observers>
            <stackday>
                <class>stackday/observer</class>
                <method>updateInventoryOnProduct</method>
                <type>singleton</type>
            </stackday>
        </observers>
    </catalog_controller_product_init>
</events>

Observer:

/**
 * Update inventory on product.
 * 
 * @param Varien_Event_Observer $observer Observer.
 *
 * @throws Exception
 * 
 * @return void
 */
public function updateInventoryOnProduct(Varien_Event_Observer $observer)
{
    /** @var Mage_Catalog_Model_Product $product */
    $product = $observer->getProduct();

    if ($product->getTypeId() != Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE) {
        return;
    }

    $childProducts = Mage::getModel('catalog/product_type_configurable')
        ->getUsedProducts(null, $product);

    foreach ($childProducts as $child) {
        /** @var Mage_Catalog_Model_Product $child */

        /** @var Mage_CatalogInventory_Model_Stock_Item $stock */
        $stock = $child->getStockItem();

        if ($stock->getQty() == 0) {
            $stock->setQty(27);
            $stock->setIsInStock(1);
            $stock->save();
        }
    }
}

But even with this code, the product may be displayed as Out of Stock - as there is an indexer in play here. You would have to run cataloginventory_stock indexer programmatically after updating stock item.

It looks to me like qty in cataloginventory_stock_item is the source of truth, and qty in cataloginventory_stock_status is the cached/indexed value. Rebuilding the Stock Status index appears to always copy the value from cataloginventory_stock_item into cataloginventory_stock_status. If there are multiple websites, there will also be a record for each product_id/website_id combo in cataloginventory_stock_status. This suggests the possibility of different stock levels per website, but as far as I can tell the qty value is always the same for each one.

I think this may be the beginnings of a multi-stock feature as suggested by Marius in this post: The purpose of the table cataloginventory_stock