how to get custom attributes in rest search api?

Question

In order rest api http://127.0.0.1/mag/index.php/rest/V1/orders/1 it shows custom attributes

 "payment_additional_info": [
        {
            "key": "method_title",
            "value": "Check / Money order"
        }
    ],
    "applied_taxes": [],
    "item_applied_taxes": [],
    "preorder": "1"

while in order search rest api http://127.0.0.1/mag/index.php/rest/V1/orders/?searchCriteria[filter_groups][0][filters][0][field]=entity_id&searchCriteria[filter_groups][0][filters][0][value]=1

it did not shows custom attribute

"payment_additional_info": [
                {
                    "key": "method_title",
                    "value": "Check / Money order"
                }
            ],
            "applied_taxes": [],
            "item_applied_taxes": []
//did not show preorder which is custom attribute 

Answer 1

Make extension_attributes.xml in vendor/module/etc add this in your extension_attributes.xml file

<?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="your attribute code" type="string" />
</extension_attributes>

Make di.xml in vendor/module/etc add this in your di.xml file

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="Magento\Sales\Api\OrderRepositoryInterface">
    <plugin name="plugin name"
            type="vendor\module\Plugin\yourplugin" />
</type>
</config>

Make OrderRepositoryPlugin.php in vendor/module/Plugin add this in your file

<?php

namespace vendour\module\Plugin;
use Magento\Sales\Api\Data\OrderExtensionFactory;
use Magento\Sales\Api\Data\OrderExtensionInterface;
use Magento\Sales\Api\Data\OrderInterface;
use Magento\Sales\Api\Data\OrderSearchResultInterface;
use Magento\Sales\Api\OrderRepositoryInterface;
/**
 * Class OrderRepositoryPlugin
 */
class OrderRepositoryPlugin
{
    /**
     * Order feedback field name
     */
    const FIELD_NAME = 'your_attribute_code';
    /**
     * Order Extension Attributes Factory
     *
     * @var OrderExtensionFactory
     */
    protected $extensionFactory;
    /**
     * OrderRepositoryPlugin constructor
     *
     * @param OrderExtensionFactory $extensionFactory
     */
    public function __construct(OrderExtensionFactory $extensionFactory)
    {
        $this->extensionFactory = $extensionFactory;
    }
    /**
     * Add "customer_feedback" extension attribute to order data object to make it accessible in API data
     *
     * @param OrderRepositoryInterface $subject
     * @param OrderInterface $order
     *
     * @return OrderInterface
     */
    public function afterGet(OrderRepositoryInterface $subject, OrderInterface $order)
    {
        $attr = $order->getData(self::FIELD_NAME);
        $extensionAttributes = $order->getExtensionAttributes();
        $extensionAttributes = $extensionAttributes ? $extensionAttributes : $this->extensionFactory->create();
        $extensionAttributes->setYourAttributeCode($attr);
        $order->setExtensionAttributes($extensionAttributes);
        return $order;
    }
    /**
     * Add "customer_feedback" extension attribute to order data object to make it accessible in API data
     *
     * @param OrderRepositoryInterface $subject
     * @param OrderSearchResultInterface $searchResult
     *
     * @return OrderSearchResultInterface
     */
    public function afterGetList(OrderRepositoryInterface $subject, OrderSearchResultInterface $searchResult)
    {
        $orders = $searchResult->getItems();
        foreach ($orders as &$order) {
            $attr = $order->getData(self::FIELD_NAME);
            $extensionAttributes = $order->getExtensionAttributes();
            $extensionAttributes = $extensionAttributes ? $extensionAttributes : $this->extensionFactory->create();
            $extensionAttributes->setYourAttributeCode($attr);
            $order->setExtensionAttributes($extensionAttributes);
        }
        return $searchResult;
    }
}

run in command line:

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
php bin/magento cache:flush