keep cart contents on login in Magento 1.8

Question

When the guest-User logs-in at checkout, the current shopping cart becomes discarded in favor of the persisted one. This is highly annoying.

I would rather have the carts to be merged.

I assume I need an observer to monitor the login action and store the current cart to merge it in the end with the persisted cart.

1. Does there exist some plugin that can do that for me?
2. Which events should i observe to gain access to both the to be discarded cart and the old persisted one
3. Do you have negative experience with this kind of "black magic"?

Answer 1

I haven't checked this in a while, but from what I remember, the guest cart should be automatically merged with the previous cart upon login.
The customer_login event is observed in the checkout module by the method Mage_Checkout_Model_Observer::loadCustomerQuote that calls Mage_Checkout_Model_Session::loadCustomerQuote that looks like this:

public function loadCustomerQuote()
{
    if (!Mage::getSingleton('customer/session')->getCustomerId()) {
        return $this;
    }
    Mage::dispatchEvent('load_customer_quote_before', array('checkout_session' => $this));
    $customerQuote = Mage::getModel('sales/quote')
        ->setStoreId(Mage::app()->getStore()->getId())
        ->loadByCustomer(Mage::getSingleton('customer/session')->getCustomerId());
    if ($customerQuote->getId() && $this->getQuoteId() != $customerQuote->getId()) {
        if ($this->getQuoteId()) {
            $customerQuote->merge($this->getQuote())
                ->collectTotals()
                ->save();
        }
        $this->setQuoteId($customerQuote->getId());
        if ($this->_quote) {
            $this->_quote->delete();
        }
        $this->_quote = $customerQuote;
    } else {
        $this->getQuote()->getBillingAddress();
        $this->getQuote()->getShippingAddress();
        $this->getQuote()->setCustomer(Mage::getSingleton('customer/session')->getCustomer())
            ->setTotalsCollectedFlag(false)
            ->collectTotals()
            ->save();
    }
    return $this;
}

Notice the line if ($customerQuote->getId() && $this->getQuoteId() != $customerQuote->getId()) {. This means that if there is a current quote object different from the customer quote the quotes should be merged $customerQuote->merge($this->getQuote()).
Check if you don't have any extensions that might interfere with this merging process.
Or you can start debugging in this last method to see why the cart merging does not happen for you.