Magento – Grouped Product can’t add any item to cart if any other item is out of stock

Unfortunately, you are not missing a step here. Normally, people would think that the following two configuration options are enough to make it happen:

1. Set System - Configuration - Catalog - Inventory - Stock Options - Display Out of Stock Products to Yes.
2. Set System - Configuration - Catalog - Inventory - Product Stock Options - Backorder to Allow Qty Below 0.

^{You of course also have to make sure that the same settings are set in the specific product or that the product is configured to use the config settings.}

Unfortunately, this is not true. So that it really works, the availability of the product has to be set to In Stock. Otherwise, Magento will not show the add to cart button and will not let you buy the product. This alone would maybe not be a problem if you could configure Magento not to set the product to Out of Stock each time it is ordered and falls below the Qty for Item's Status to Become Out of Stock barrier. But this is not possible. For the smart guys out here: No, it is also not possible to set this setting to a quantity below 0 via the admin settings.

Possibilities:

1. You could fix the issue that the product is set to Out of Stock by fixing the config entry in the database: UPDATE `core_config_data` SET `value` = '-9999' WHERE `path` = 'cataloginventory/item_options/min_qty';. Although this then applies to all products, not only to the ones which are configured for backorders.
2. You could try one of the "Preorder" or "Backorder" extensions.
3. You could set the product to In Stock manually each time such an order occurs (of course only possible for very rare cases).

I found a solution which does not need an observer.

I had to do a rewrite to Mage_Sales_Model_Quote. More specific the method addProductAdvanced()

Bad news: You can not simply use parent::addProductAdvanced() and then do your own stuff. You have to copy the original code and fill it up with your code.

Here is what I did (look out for the comment starting with /********):

public function addProductAdvanced(Mage_Catalog_Model_Product $product, $request = null, $processMode = null) {
    if ($request === null) {
        $request = 1;
    }
    if (is_numeric ( $request )) {
        $request = new Varien_Object ( array (
                'qty' => $request 
        ) );
    }
    if (! ($request instanceof Varien_Object)) {
        Mage::throwException ( Mage::helper ( 'sales' )->__ ( 'Invalid request for adding product to quote.' ) );
    }

    $cartCandidates = $product->getTypeInstance ( true )->prepareForCartAdvanced ( $request, $product, $processMode );

    /**
     * Error message
     */
    if (is_string ( $cartCandidates )) {
        return $cartCandidates;
    }

    /**
     * If prepare process return one object
     */
    if (! is_array ( $cartCandidates )) {
        $cartCandidates = array (
                $cartCandidates 
        );
    }

    $parentItem = null;
    $errors = array ();
    $items = array ();
    foreach ( $cartCandidates as $candidate ) {
        // Child items can be sticked together only within their parent
        $stickWithinParent = $candidate->getParentProductId () ? $parentItem : null;
        $candidate->setStickWithinParent ( $stickWithinParent );
        $item = $this->_addCatalogProduct ( $candidate, $candidate->getCartQty () );

        /******** own modification for custom option GROUPSKU*/
        if($product->isGrouped()){
            $additionalOptions = array (
                    'options' => array (
                            'label' => 'GROUPSKU',
                            'value' => $product->getSku ()
                    )
            );
            $item->addOption ( new Varien_Object ( array (
                    'product' => $item->getProduct (),
                    'code' => 'additional_options',
                    'value' => serialize ( $additionalOptions )
            ) ) );
        }
        /******** own modification end*/

        if ($request->getResetCount () && ! $stickWithinParent && $item->getId () === $request->getId ()) {
            $item->setData ( 'qty', 0 );
        }
        $items [] = $item;

        /**
         * As parent item we should always use the item of first added product
         */
        if (! $parentItem) {
            $parentItem = $item;
        }
        if ($parentItem && $candidate->getParentProductId ()) {
            $item->setParentItem ( $parentItem );
        }

        /**
         * We specify qty after we know about parent (for stock)
         */
        $item->addQty ( $candidate->getCartQty () );

        // collect errors instead of throwing first one
        if ($item->getHasError ()) {
            $message = $item->getMessage ();
            if (! in_array ( $message, $errors )) { // filter duplicate messages
                $errors [] = $message;
            }
        }
    }
    if (! empty ( $errors )) {
        Mage::throwException ( implode ( "\n", $errors ) );
    }

    Mage::dispatchEvent ( 'sales_quote_product_add_after', array (
            'items' => $items 
    ) );

    return $item;
}

This way all the products that are added to the cart using a grouped product now have the custom option.