API: add 3 extra attributes

[hanhoe]

Hello,

I have in the table "sales_order_item" 3 extra rows, which is for special attribute when a product is buying.

How can I access this via API?

I made a call at "rest/V1/orders/items/ID" and get the rows of the order. But this extendet attributes are not included.

Thank you!

[hanhoe]

I tried to write a new API Endpoint, but always has the error:

'%fieldName is a required field":

etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
  <preference for="Gedo\Color\Api\ColorInterface" type="Gedo\Color\Model\Color" />
</config>

etc/webapi.xml

<?xml version="1.0"?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
<!-- Routing define -->
<route url="/V1/color/getcolor/:getcolor" method="GET">
 <service class="Gedo\Color\Api\ColorInterface" method="getcolor"/>
 <resources>
  <resource ref="anonymous"/>
 </resources>
</route>
</routes>

Api/ColorInterface.php:

<?php

namespace Gedo\Color\Api;

interface ColorInterface
{
	/**
     * Return the special color
     *
     * @param int $itemId
     * @return mixed
     */
public function getcolor($itemId);
}

Model/Color.php

<?php

namespace Gedo\Color\Model;

use Gedo\Color\Api\ColorInterface;

protected $_resourceConnection;
protected $_connection;
public function __construct(
    \Magento\Framework\App\ResourceConnection $resourceConnection
) {
    $this->_resourceConnection = $resourceConnection;
}


class Color implements ColorInterface
{
	/**
     * Return the special color
     *
     * @param string $itemId
     * @return mixed
     */	
public function getcolor($itemId) {

	$this->_connection = $this->_resourceConnection->getConnection();
	
	$query = "Select ral_color_picker, ral_color_text, abgetont from sales_order_item where item_id == " . $itemId ;     
	
	$collection = $this->_connection->fetchAll($query);
	return $collection;
 }

}

I made the GET to http://magento2.loc/rest/V1/color/getcolor/56

but get always "%fieldName is a required field"