Magento – How to display related products on the cart page (checkout)

Ok, I finally can add the related_product to the cart. In the code/core/Mage/Checkout/Model/Cart.php file are two functions for adding products into the cart. The first is called public function addProduct($productInfo, $requestInfo=null), the second function is public function addProductsByIds($productIds).

The related_product is added by id, the others are added with the addProduct() function. The adding by id does not seem to work (at least for me it did not. Although magento used the unaltered Cart.php file from 1.7.0.2.

I solved this by altering the Cart.php file like this:

 /**
 * Add product to shopping cart (quote)

 * This is the original function pasted from
 * Mage/Checkout/Model/Cart.php line 245
 * The function is working and adding products to the cart

 *
 * @param   int|Mage_Catalog_Model_Product $productInfo
 * @param   mixed $requestInfo
 * @return  Mage_Checkout_Model_Cart
 */
public function addProduct($productInfo, $requestInfo=null)
{
    $product = $this->_getProduct($productInfo);
    $request = $this->_getProductRequest($requestInfo);

    $productId = $product->getId();

    if ($product->getStockItem()) {
        $minimumQty = $product->getStockItem()->getMinSaleQty();
        //If product was not found in cart and there is set minimal qty for it
        if ($minimumQty && $minimumQty > 0 && $request->getQty() < $minimumQty
            && !$this->getQuote()->hasProductId($productId)
        ){
            $request->setQty($minimumQty);
        }
    }

    if ($productId) {
        try {
            $result = $this->getQuote()->addProduct($product, $request);
        } catch (Mage_Core_Exception $e) {
            $this->getCheckoutSession()->setUseNotice(false);
            $result = $e->getMessage();
        }
        /**
         * String we can get if prepare process has error
         */
        if (is_string($result)) {
            $redirectUrl = ($product->hasOptionsValidationFail())
                ? $product->getUrlModel()->getUrl(
                    $product,
                    array('_query' => array('startcustomization' => 1))
                )
                : $product->getProductUrl();
            $this->getCheckoutSession()->setRedirectUrl($redirectUrl);
            if ($this->getCheckoutSession()->getUseNotice() === null) {
                $this->getCheckoutSession()->setUseNotice(true);
            }
            Mage::throwException($result);
        }
    } else {
        Mage::throwException(Mage::helper('checkout')->__('The product does not exist.'));
    }

    Mage::dispatchEvent('checkout_cart_product_add_after', array('quote_item' => $result, 'product' => $product));
    $this->getCheckoutSession()->setLastAddedProductId($productId);
    return $this;
}

/**
 * Adding products to cart by ids

 * This function is the altered function, which works for me
 * the edited lines are marked.

 *
 * @param   array $productIds
 * @return  Mage_Checkout_Model_Cart
 */
public function addProductsByIds($productIds)
{
    $allAvailable = true;
    $allAdded     = true;

    if (!empty($productIds)) {
        foreach ($productIds as $productId) {
            $productId = (int) $productId;
            if (!$productId) {
                continue;
            }
            $product = $this->_getProduct($productId);
            if ($product->getId() && $product->isVisibleInCatalog()) {
                try {
                    $this->getQuote()->addProduct($product);
                } catch (Exception $e){
                    $allAdded = false;
                }

                /**
                 * START EDIT
                 */

                /**
                * String we can get if prepare process has error
                */
                if (is_string($result)) {
                    $redirectUrl = ($product->hasOptionsValidationFail())
                        ? $product->getUrlModel()->getUrl(
                            $product,
                            array('_query' =>array                                   ('startcustomization' => 1))
                            )
                        : $product->getProductUrl();
                    $this->getCheckoutSession()->setRedirectUrl($redirectUrl);
                    if ($this->getCheckoutSession()->getUseNotice() === null) {
                        $this->getCheckoutSession()->setUseNotice(true);
                    }
                    Mage::throwException($result);
                }
                Mage::dispatchEvent('checkout_cart_product_add_after', array('quote_item' => $result, 'product' => $product));
                $this->getCheckoutSession()->setLastAddedProductId($productId);

                /**
                 * STOP EDIT
                 */

            } else {
                $allAvailable = false;
            }
        }

        if (!$allAvailable) {
            $this->getCheckoutSession()->addError(
                Mage::helper('checkout')->__('Some of the requested products are unavailable.')
            );
        }
        if (!$allAdded) {
            $this->getCheckoutSession()->addError(
                Mage::helper('checkout')->__('Some of the requested products are not available in the desired quantity.')
            );
        }
    }
    return $this;
}

So i copied the lines from the other function. Does anyone think this will have some drawbacks? For me it looks like this introduces another check if everything works correct by examining the return string. But i don't know if dispatching the event might have some negative effects. As i stated, I'm new to magento and would like to know if this is solid solution.

Does anyone know, why the functions are different? Because they do basically the same. The only difference is, that the second function can add more than product. Maybe in this case dispatching the event is slow?