Magento 2 how add extension attribute to order item

Question

I need to save the extension attribute for Order item. I have tried using plugin with below code, but it is not working for me. I have placed a log inside the method but the method is not working.

<type name="Magento\Sales\Api\OrderItemRepositoryInterface">
    <plugin name="order_item_extension_attr" type="Vendor\CustomModule\Plugin\Model\Test"/>
</type>

public function afterGet(
        \Magento\Sales\Api\OrderItemRepositoryInterface $orderItem, 
        \Magento\Sales\Api\Data\OrderItemExtensionInterface $extension = null
    ) {

}

How to save and get order item extension attribute?

Edited: Used an observer after order saved, stored the order and item extension attributes in db. Which is working fine, but showing the order item values are not showing up in the order view page.

Answer 1

add below plugin to your di.xml

<type name="Magento\Sales\Api\OrderRepositoryInterface">
    <plugin name="get_order_comment" type="Vendor\Module\Plugin\Yourplugin"/>
</type>

Now create below plugin here

Vendor\Module\Plugin\Yourplugin

class Yourplugin {

    protected $_orderExtensionFactory;

    public function __construct(
        \Magento\Sales\Api\Data\OrderExtensionFactory $orderExtensionFactory
    ) {
        $this->_orderExtensionFactory = $orderExtensionFactory;
    }

    public function afterGet(
        \Magento\Sales\Api\OrderRepositoryInterface $subject,
        \Magento\Sales\Api\Data\OrderInterface $order
    ) {
        $extensionAttributes = $order->getExtensionAttributes();
        $orderExtension = $extensionAttributes ? $extensionAttributes : $this->_orderExtensionFactory->create();

        $comment = {write the logic to get the comment. e.g. $order->getComment()}
        $orderExtension->setComment($comment);

        $order->setExtensionAttributes($orderExtension);

        return $order;
    }
}

Answer 2

1. If you want to use extension attribute then you must declare extension attribute at app/code/YOurModule/{Modulename}/etc/extension_attributes.xml.

Create extension_attributes.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Sales\Api\Data\OrderItemInterface">
        <attribute code="{fieldname}" type="string" />
    </extension_attributes>
</config>

Assume that field type is varchar/text.

2. Create after plugin on save() method of Magento\Sales\Api\OrderRepositoryInterface while save Order using Magento\Sales\Api\OrderRepositoryInterface:save method.