Add Order Attribute to Custom Attributes on Magento 2 Rest API?

Step 1: Create new column and set value for the existing order

Firstly, you need to create a new column in table sales_order in your installer file 

 $setup->getConnection()->addColumn($setup->getTable('sales_order'), 'order_attribute', 'varchar(1)');

In this post, we named it “order_attribute”.

Step 2: Create extension_attributes.xml file

Next, you have to create a file {vendor}\{module}\etc\extension_attributes.xml in the extension folder with following content:

<?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\OrderInterface">
    <attribute code="order_attribute" type="string" />
</extension_attributes>
</config>

You have to create file extension_attributes.xml to add the custom attribute because Magento 2 doesn’t accept to add a new field in the response on the Rest API

Step 3: Create observe

In this step, you need to create observer for event ‘sales_order_load_after’ through the file {vendor}\{module}\etc\events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"   xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="sales_order_load_after">
    <observer name="sales_order_load_order_attribute" instance="{Vendor}\{Module}\Observer\Sales\OrderLoadAfter" />
</event>
</config>

 

Step 4: Create a file to handle this event

Create a file {vendor}\{module}\Observer\Sales\OrderLoadAfter.php to handle the event ‘sales_order_load_after’ which is declared above

<?php
namespace {Vendor}\{Module}\Observer\Sales;
use Magento\Framework\Event\ObserverInterface;
class OrderLoadAfter implements ObserverInterface
{
public function execute(\Magento\Framework\Event\Observer $observer)
{
    $order = $observer->getOrder();
    $extensionAttributes = $order->getExtensionAttributes();
    if ($extensionAttributes === null) {
        $extensionAttributes = $this->getOrderExtensionDependency();
    }
    $attr = $order->getData('order_attribute');
    $extensionAttributes->setOrderAttribute($attr);
    $order->setExtensionAttributes($extensionAttributes);
}
private function getOrderExtensionDependency()
{
    $orderExtension = \Magento\Framework\App\ObjectManager::getInstance()->get(
        '\Magento\Sales\Api\Data\OrderExtension'
    );
    return $orderExtension;
}
}

last step is executing these commands

php bin/magento setup:upgrade
php bin/magento cache:clean
php bin/magento cache:flush