How to merge the non-logged in cart items with logged in customer cart items which is added from favorite's in Magento 2

Question

I am facing an issue with cart merging

this is the scenario.

1. I logged in to my account, i added one product to favorites then from favorites page i added that product to cart
2. Next i logged out, added the same product from PDP or PLP as a guest.
3. Then i logged in to the same account.

Now instead of showing the single product with quantity 2, it showing like two products in the cart with quantity 1 and 1.

I need to show like single product with updated qty. How can i achieve this.

Answer 1

magento merge the guest and customer quote from the class

vendor/magento/module-quote/Model/Quote.php

find the merge function and which should be on line number 2401 and replace the function

public function merge(Quote $quote)
{
    $this->_eventManager->dispatch(
        $this->_eventPrefix . '_merge_before',
        [$this->_eventObject => $this, 'source' => $quote]
    );

    foreach ($quote->getAllVisibleItems() as $item) {
        $found = false;
        foreach ($this->getAllItems() as $quoteItem) {
            if ($quoteItem->compare($item)) {
                $quoteItem->setQty($quoteItem->getQty() + $item->getQty());
                $this->itemProcessor->merge($item, $quoteItem);
                $found = true;
                break;
            }
        }

        if (!$found) {
            $newItem = clone $item;
            $this->addItem($newItem);
            if ($item->getHasChildren()) {
                foreach ($item->getChildren() as $child) {
                    $newChild = clone $child;
                    $newChild->setParentItem($newItem);
                    $this->addItem($newChild);
                }
            }
        }
    }

    /**
     * Init shipping and billing address if quote is new
     */
    if (!$this->getId()) {
        $this->getShippingAddress();
        $this->getBillingAddress();
    }

    if ($quote->getCouponCode()) {
        $this->setCouponCode($quote->getCouponCode());
    }

    $this->_eventManager->dispatch(
        $this->_eventPrefix . '_merge_after',
        [$this->_eventObject => $this, 'source' => $quote]
    );

    return $this;
}

with the below customized function

public function merge(Quote $quote)
{
    $this->_eventManager->dispatch(
        $this->_eventPrefix . '_merge_before',
        [$this->_eventObject => $this, 'source' => $quote]
    );

    foreach ($quote->getAllVisibleItems() as $item) {
        $found = false;
        foreach ($this->getAllItems() as $quoteItem) {
            /* resolving quote merge bug issue @ritesh24feb2021 
            * this will merge the products if product id is same 
            * please note : this will not check for custom options now 
            */
            $sameProductId = false;
            if ($quoteItem->getProductId() == $item->getProductId()) {
                $sameProductId = true;
            }

            if ($quoteItem->compare($item) || $sameProductId) {
                $quoteItem->setQty($quoteItem->getQty() + $item->getQty());
                $this->itemProcessor->merge($item, $quoteItem);
                $found = true;
                break;
            }
        }

        if (!$found) {
            $newItem = clone $item;
            $this->addItem($newItem);
            if ($item->getHasChildren()) {
                foreach ($item->getChildren() as $child) {
                    $newChild = clone $child;
                    $newChild->setParentItem($newItem);
                    $this->addItem($newChild);
                }
            }
        }
    }

    /**
     * Init shipping and billing address if quote is new
     */
    if (!$this->getId()) {
        $this->getShippingAddress();
        $this->getBillingAddress();
    }

    if ($quote->getCouponCode()) {
        $this->setCouponCode($quote->getCouponCode());
    }

    $this->_eventManager->dispatch(
        $this->_eventPrefix . '_merge_after',
        [$this->_eventObject => $this, 'source' => $quote]
    );

    return $this;
}

Please Note : i am not advising to change the code of vendor directory directly . if it works for you , you can overwrite this class in your own module , i would be happy to assist further if it solves your issue .