How to get the custom order attributes in default order Api response in magento 2.3

Question

I have created a new custom order attribute named delivery_date and shown the same in sales order grid but i am not getting the custom attribute in my order Api response.

The error I am getting is Fatal error: Uncaught Error: Call to undefined method Magento\Sales\Api\Data\OrderExtension::setTipAndTrickAttribute()

Please help.

app/code/Amos/CustomOrder/etc/di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2013-2017 Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <virtualType name="Magento\Sales\Model\ResourceModel\Order\Grid" type="Magento\Sales\Model\ResourceModel\Grid">
        <arguments>
            <argument name="columns" xsi:type="array">
                <item name="delivery_date" xsi:type="string">sales_order.delivery_date</item>
                <item name="no_of_days" xsi:type="string">sales_order.no_of_days</item>
                <item name="no_of_crew" xsi:type="string">sales_order.no_of_crew</item>
            </argument>
        </arguments>
    </virtualType>    
</config>

app/code/Amos/CustomOrder/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_delivery_date" instance="Magestore\TipAndTrick\Observer\Sales\OrderLoadAfter" />
</event>
</config>

Amos/CustomOrder/etc/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\OrderInterface">
    <attribute code="delivery_date" type="string" />
</extension_attributes>
</config>  

Amos/CustomOrder/Observer/Sales/OrderLoadAfter.php

<?php
namespace Amos\CustomOrder\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('delivery_date');
    $extensionAttributes->setTipAndTrickAttribute($attr);
    $order->setExtensionAttributes($extensionAttributes);
}
private function getOrderExtensionDependency()
{
    $orderExtension = \Magento\Framework\App\ObjectManager::getInstance()->get(
        '\Magento\Sales\Api\Data\OrderExtension'
    );
    return $orderExtension;
}
}

Answer 1

To answer your question about the error you're using the wrong magic function. Your magic functions for that attribute is setDeliveryDate().

You also need to make sure your events.xml has the right class for the observer.

<observer name="sales_order_load_delivery_date" instance="Magestore\TipAndTrick\Observer\Sales\OrderLoadAfter" />

While your observer class is: Amos\CustomOrder\Observer\Sales\OrderLoadAfter

When you are using example material try not to forget to change the class, namespace and functions names among other things when you need to. You may also need an order repository plugin to actually get it into the API response.

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