how to cancel certain order item rather than cancel whole order?

Question

I need a function which is order cancel based on the item. Current scenario is that Magento does cancel whole order.
What I want:
A customer placed 2 ordered items. And after placed order, customer wants to cancel 1 item,so in this situation which parameter I should consider and which approach I have to go ?

I found some below code in Magento core files,if someone check and give me some hint about cancel order item rather than whole order,it will be appreciated.

app\code\core\Mage\Sales\Model\Order.php

 public function registerCancellation($comment = '', $graceful = true)
    {
        if ($this->canCancel() || $this->isPaymentReview()) {
            $cancelState = self::STATE_CANCELED;
            foreach ($this->getAllItems() as $item) {
                if ($cancelState != self::STATE_PROCESSING && $item->getQtyToRefund()) {
                    if ($item->getQtyToShip() > $item->getQtyToCancel()) {
                        $cancelState = self::STATE_PROCESSING;
                    } else {
                        $cancelState = self::STATE_COMPLETE;
                    }
                }
                $item->cancel();
            }

            $this->setSubtotalCanceled($this->getSubtotal() - $this->getSubtotalInvoiced());
            $this->setBaseSubtotalCanceled($this->getBaseSubtotal() - $this->getBaseSubtotalInvoiced());

            $this->setTaxCanceled($this->getTaxAmount() - $this->getTaxInvoiced());
            $this->setBaseTaxCanceled($this->getBaseTaxAmount() - $this->getBaseTaxInvoiced());

            $this->setShippingCanceled($this->getShippingAmount() - $this->getShippingInvoiced());
            $this->setBaseShippingCanceled($this->getBaseShippingAmount() - $this->getBaseShippingInvoiced());

            $this->setDiscountCanceled(abs($this->getDiscountAmount()) - $this->getDiscountInvoiced());
            $this->setBaseDiscountCanceled(abs($this->getBaseDiscountAmount()) - $this->getBaseDiscountInvoiced());

            $this->setTotalCanceled($this->getGrandTotal() - $this->getTotalPaid());
            $this->setBaseTotalCanceled($this->getBaseGrandTotal() - $this->getBaseTotalPaid());

            $this->_setState($cancelState, true, $comment);
        } elseif (!$graceful) {
            Mage::throwException(Mage::helper('sales')->__('Order does not allow to be canceled.'));
        }
        return $this;
    }

Answer 1

The best practice is cancel current and create new order.

Here is why: Magento uses qoutes and all functionalities for rebuilding totals, applying cart rules is in quote model, after order placed - all info just goes from quite to order. So if you will delete order item - you will have to change manually all totals, taxes, inventory of product from item, recalculate cart rules and many other things.