I am tring to add multiples of x products in to cart, that is working fine as expected after check out i want to decrement the product quantity, how can i do that? I am using an event observer for this.

Example:- i have a product which is $2 and i am adding this single product to the cart, in the cart it will show customergivenQty(1) x multiplier_value(5) x price($2) so the total amount is $10 and the quantity which i want to reduce is customergivenQty(1) x multiplier_value(5)

class Custom_Module_Modulename_Model_Observer
    public function _construct()

    public function getNewPrice()

        $login  = Mage::getSingleton('customer/session')->isLoggedIn();
        $roleId = Mage::getSingleton('customer/session')->getCustomerGroupId();
        $userrole   = Mage::getSingleton('customer/group')->load($roleId)->getData('customer_group_code');
        $userrole   = strtolower($userrole);
        $quote = Mage::getSingleton('checkout/session')->getQuote();
        $cartItems = $quote->getAllVisibleItems();
        foreach ($cartItems as $item) {
            $productId = $item->getProductId();
            $product = Mage::getModel('catalog/product')->load($productId);
        $batch_qty = $product->getBatchQty();
        $actualPrice = $product->getPrice();
        $specialPrice = $product->getFinalPrice();
        if (isset($batch_qty) && $userrole=="retailer") {
            if (isset($specialPrice)) {
                $newprice = $specialPrice*$batch_qty;
            } else {

                $newprice = $actualPrice*$batch_qty;

        } else {
            $newprice= $actualPrice;

        return $newprice;

    public function updatePrice($observer)
        $event = $observer->getEvent();
        $product = $event->getProduct();
        $quote_item = $event->getQuoteItem();
        $new_price = $this->getNewPrice();


config.xml file look like this

  • So you want to decrease the quantity of the product each time a product is added to the cart? May 7, 2015 at 17:10
  • No, I want to decrease the quantity after pacing an successful order. my be after payment success. Above code works fine for the price update, same way i need to update decreasing quantity
    – Naveenbos
    May 8, 2015 at 6:14
  • Magento does that automatically. You can configure this behavior under system>configuration>catalog inventory May 8, 2015 at 6:32

1 Answer 1


Finally i find out a solution for this.Below shows the function in Observer.php

public function updateQuantity($observer)
        $roleId     = Mage::getSingleton('customer/session')->getCustomerGroupId();
        $userrole   = Mage::getSingleton('customer/group')->load($roleId)->getData('customer_group_code');
        $userrole   = strtolower($userrole);
        $orderId    = Mage::getSingleton('checkout/session')->getLastRealOrderId();
        $order      = Mage::getModel('sales/order')->load($orderId, 'increment_id');
        $ordered_items = $order->getAllItems();            

        if ($userrole == "retailer") {
            foreach ($ordered_items as $item) {
                //item detail                
                $orderedQty = $item->getQtyOrdered(); //ordered qty of item
                $totalQty = $item->getProduct()->getStockItem()->getQty();
                $actualQty = $totalQty + $orderedQty;
                $batchQty = $item->getProduct()->getBatchQty();
                $newQty = $actualQty-$orderedQty*$batchQty;                

Config.xml file i have did this.


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