Question

I’ve been looking over Magento’s code (1.6.2 Community Edition), and OMG is it a terribly executed mess, but I won’t rant here about that.

Looking at this code:

// Delete error from item and its quote, if it was set due to qty lack
$this->_removeErrorsFromQuoteAndItem($quoteItem, Mage_CatalogInventory_Helper_Data::ERROR_QTY);

Which is in Mage_CatalogInventory_Model_Observer class in
/app/code/core/Mage/CatalogInventory/Model/Observer.php
Line: 489

This produces a peculiar bug to me, try this (make sure “no backorders” is set in backend):

1. Add a product to the cart
2. Add another product to the cart.
3. Go in admin and change first product quantity such that the requested quantity in the cart is not enough.
4. Go back to shopping cart page.

Result
The first product added shows the “requested item not available in this quantity” error, but the Checkout methods are still shown ! Continuing forward, eventually gives an ugly js alert saying “Not all items are available in the requested qty”.

Digging deeper I see that it seems every quote item (cart item) added to the quote (cart) resets the error state for the whole quote.

Is this intentional ? Have I stumbled upon a genuine bug ?
What does this useless comment mean ?
Thanks in advance.

EDIT: See answer below for workaround and explanation.

Answer 1

I have to post a follow up, My core hack had an unfortunate side effect (another bug):

1. Add item to cart with certain quantity (make sure it is stock).
2. Go in admin panel and reduce stock so that the remaining quantity is no longer.
3. Go back to the cart page and refresh, you get the expected error.
4. Try to update the cart and reduce the requested quantity to that which is in fact available, you will notice that the update does not work and the error from #3 is redisplayed.

So apparently this particular logic is necessary for that scenario.

The actual flaw is that the quote error info collection does not differentiate between quote items. So the _removeErrorsFromQuoteAndItem function removes all quantity related errors from the cart including those that came from another quote item (that is still in error).

I found a workaround that works without any core hacks, attach this event observer to the sales_quote_item_qty_set_after event.

public function reinitQuoteErrorState(Varien_Event_Observer $observer)
{
    $item = $observer->getEvent()->getItem();
    /** @var $item Mage_Sales_Model_Quote_Item */

    $quote = $item->getQuote();
    /** @var $quote Mage_Sales_Model_Quote */

    // Quote not loaded, do nothing since our changes are transient
    if (!$quote) return;

    // Quote already has error state, nothing to do.
    if ($quote->getHasError()) return;

    foreach ($quote->getAllItems() as $quoteItem)
    {
        if ($errorItems = $quoteItem->getErrorInfos())
        {
            foreach ($errorItems as $errorItem)
            {
                if ($errorItem['code'] == Mage_CatalogInventory_Helper_Data::ERROR_QTY)
                {
                    $quote->addErrorInfo(
                        'error',
                        'cataloginventory',
                        Mage_CatalogInventory_Helper_Data::ERROR_QTY,
                        Mage::helper('cataloginventory')->__('Not all products are available in the requested quantity')
                    );

                    return;
                }
            }
        }
    }
}

Note: The module that has this event handler must add to the tag of the module XML file in /app/etc/modules/ (eg: /app/etc/modules/MyCompany_MyModule.xml) to ensure that this handler is called after Mage_CatalogInventory_Model_Observer::checkQuoteItemQty which has the bug.

The real solution is to fix the quote errorInfo system to maintain quote item identity, but this workaround will do without any core hacks.

<?xml version="1.0" encoding="utf-8"?>
<config>
    <modules>
        <MyCompany_MyModule>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Mage_CatalogInventory />
            </depends>
        </MyCompany_MyModule>
    </modules>
</config>