Q:

Magento 2 copy value from custom attribute to order item

I have created a custom attribute for products. And I have also created an extra column in sales_order_item. How can I copy the value from this custom attribute to the new column in sales_order_item when an order is placed?

A:

You can use plugin to add custom attribute to sales_order_item when order is placed.
create a di.xml on under etc/di.xml.
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd" >
    <type name='Magento\Quote\Model\Quote\Item\ToOrderItem'>
        <plugin name='AddOrderItemPlugin' type='Vendorname\Module\Plugin\Model\Quote\Item\ToOrderItem' sortOrder='99'/>
    </type>
</config>

Now add a file Vendorname\Module\Plugin\Model\Quote\Item\ToOrderItem.php
<?php 
namespace Vendorname\Modulename\Plugin\Model\Quote\Item;

    class ToOrderItem {

        /**
         *
         * @var type \Magento\Catalog\Model\Product
         */
        protected $productRepository;

        /**
         * @param \Magento\Catalog\Model\Product $productRepository 
         */
        public function __construct(
        \Magento\Catalog\Model\Product $productRepository 
        ) {
            $this->productRepository = $productRepository;
        }

        /**
         * 
         * @param \Magento\Quote\Model\Quote\Item\ToOrderItem $subject
         * @param \Vendorname\Modulename\Plugin\Model\Quote\Item\callable $proceed
         * @param \Magento\Quote\Model\Quote\Item\AbstractItem $item
         * @param type $additional
         * @return type
         */
        public function aroundConvert(
        \Magento\Quote\Model\Quote\Item\ToOrderItem $subject, callable $proceed, \Magento\Quote\Model\Quote\Item\AbstractItem $item, $additional = []
        ) {

            $orderItem = $proceed($item, $additional);
            $productId = $item->getProduct()->getId();
            $product = $this->productRepository->load($productId);
            $supplier = $product->getSuppliers();//my custom product attribute
            $orderItem->setSupplierId($supplier);//saving into orde item in `supplier_id` column
            return $orderItem;
        }

    }

