Update magento cart total before saving order

Question

I need to remove some products from cart after the customer press the "Place order" button. I can remove the products with a custom event and observer and update the cart products, but i cant update the cart total. In the observer i try to update the price with:

    $granTotal = $cart->getGrandTotal() - $cart->getShippingAmount();
    $cart->setGrandTotal($GranTotal - $sum)->save();

    $baseGranTotal = $cart->getBaseGrandTotal() - $cart->getShippingAmount();
    $cart->setBaseGrandTotal($baseGranTotal - $sum)->save();

    $cart->setTotalsCollectedFlag(false)->collectTotals();
    $cart->save();

But the total of the cart don't change. I find that the total is stored in the cart address too and used to set the cart total in Mage_Sales_Model_Quote collectTotals() function. Here:

$this->setSubtotal((float) $this->getSubtotal() + $address->getSubtotal());
$this->setBaseSubtotal((float) $this->getBaseSubtotal() + $address->getBaseSubtotal());

Then i tried to change the address total before calling the collectTotals(), but the cart total still don't change.

My custom event is triggered at the beginning in Mage_Checkout_Model_Type_Onepage saveOrder() function.

I am running out of ideas of how to update the total of the cart. My observer looks like this:

public function changeCart(){
    $cart = Mage::getSingleton('checkout/session')->getQuote();
    $data = array();
    $inquiry = array();
    $sum = 0;
    //remove out of stock items to save them in custom model
    foreach ($cart->getAllItems() as $item) {
        if($item->getProduct()->getData('is_in_stock') == 0){
            $cart->removeItem($item->getItemId())->save();
            $inquiry[] = $item->getProduct();
            $data['qty'][$item->getProduct()->getId()] = $item->getData('qty');
            $sum += $item->getProduct()->getFinalPrice();
            $dobavljivost = $item->getProduct()->getResource()->getAttributeRawValue($item->getProduct()->getId(), 'dobavljivost', Mage::app()->getStore());
            Mage::getSingleton('core/session')->setDobavljivost($dobavljivost);
        }
    }

    foreach($cart->getAllAddresses() as $a){
        $a->setGrandTotal(0);;
        $a->setBaseGrandTotal(0);
        $a->setData('subtotal', 0);
        $a->setData('base_subtotal', 0);
    }

    $granTotal = $cart->getGrandTotal() - $cart->getShippingAmount();
    $cart->setGrandTotal($GranTotal - $sum)->save();

    $baseGranTotal = $cart->getBaseGrandTotal() - $cart->getShippingAmount();
    $cart->setBaseGrandTotal($baseGranTotal - $sum)->save();

    $cart->setTotalsCollectedFlag(false)->collectTotals();
    $cart->save();
}

Thank you for any help!

Answer 1

For recomputing totals of the cart you can try do this

$cart = Mage::getSingleton('checkout/session')->getQuote();

foreach ($cart->getAllAddresses() as $address)
{
   $address->unsetData('cached_items_nonnominal');
   $address->unsetData('cached_items_nominal');
}

$cart->setTotalsCollectedFlag(false);
$cart->collectTotals();