Integrity constraint error referring to quote_item table while adding product to cart

Question

I am getting the following error while trying to add product to cart in Magento 2.2.3.

[2018-05-02 09:52:43] report.CRITICAL: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (magento2.quote_item, CONSTRAINT QUOTE_ITEM_QUOTE_ID_QUOTE_ENTITY_ID FOREIGN KEY (quote_id) REFERENCES quote (entity_id) ON DELETE CASCADE), query was: INSERT INTO quote_item (product_id, store_id, is_virtual, sku, name, is_qty_decimal, weight, qty, product_type, base_cost, subscription_frequency) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) {"exception":"[object] (Zend_Db_Statement_Exception(code: 23000): SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (magento2.quote_item, CONSTRAINT QUOTE_ITEM_QUOTE_ID_QUOTE_ENTITY_ID FOREIGN KEY (quote_id) REFERENCES quote (entity_id) ON DELETE CASCADE), query was: INSERT INTO quote_item (product_id, store_id, is_virtual, sku, name, is_qty_decimal, weight, qty, product_type, base_cost, subscription_frequency) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) at C:\xampp\htdocs\mage2\vendor\magento\zendframework1\library\Zend\Db\Statement\Pdo.php:235, PDOException(code: 23000): SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (magento2.quote_item, CONSTRAINT QUOTE_ITEM_QUOTE_ID_QUOTE_ENTITY_ID FOREIGN KEY (quote_id) REFERENCES quote (entity_id) ON DELETE CASCADE) at C:\xampp\htdocs\mage2\vendor\magento\zendframework1\library\Zend\Db\Statement\Pdo.php:228)"} []

I have added a custom column in quote_item table. Not sure if this is the issue. Kindly help resolve it. TIA

EDIT:

I'm updating my custom using event - checkout_cart_product_add_after . Below is my observer update code.

<?php

namespace Addon\Recur\Observer;
use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;

class CheckoutCartProductAddAfterObserver implements ObserverInterface
{
    /**
     * @var \Magento\Framework\View\LayoutInterface
     */
    protected $_layout;
    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;
    protected $_request;

    /**
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\View\LayoutInterface $layout
     */
    public function __construct(
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\View\LayoutInterface $layout,
        \Magento\Framework\App\RequestInterface $request
     )
    {
        $this->_layout = $layout;
        $this->_storeManager = $storeManager;
        $this->_request = $request;
    }


    /**
     * Add order information into GA block to render on checkout success pages
     *
     * @param EventObserver $observer
     * @return void
     */
   public function execute(\Magento\Framework\Event\Observer $observer) {

        /* @var \Magento\Quote\Model\Quote\Item $item */
        $item = $observer->getQuoteItem();
        $post = $this->_request->getParam('frequency');
        $item->setSubscriptionFrequency($post);
        $item->save();

        $writer = new \Zend\Log\Writer\Stream(BP . '/var/log/test.log');  
        $logger = new \Zend\Log\Logger();  
        $logger->addWriter($writer);  
        $logger->info($post);
   }
}

Answer 1

I ran into the same problem when i tried to change data on quote items with the checkout_cart_product_add_after event. This event is fired after every product add, but before the cart is saved. Check the Magento\Checkout\Controller\Cart\Add line 108-113 for reference. So if you want to save the quote item on a new session there is no quote yet and there is no quote_id to reference to.

In my case, i just left the $item->save(); out, because it will be saved afterwards automatically.