REST API for updating existing order item data in Magento 2

if you need to update sales_order_item table after placing an order (while your product is based on subscription method), here you will get step by step information on how to update sales order item table data using rest api.
create or add in your existing module custom rest api in magento 2.x. 
Step1 : -  create interface file in <oodles>/<ProductRE>/Api/GetCustomProductListManagementInterface.php and paste below code

<?php
declare(strict_types=1);
namespace oodles\ProductRE\Api;
interface GetCustomProductListManagementInterface
{
  /**
     * GET for update order item Data api
     * @api
     * @return mixed|null
     */
    public function updateOrderItem(); 
}

Step 2: - create file in  app/code/oodles/ProductRE/Model/GetCustomProductListManagement.php and paste below code 
 

<?php
declare(strict_types=1);
namespace oodles\ProductRE\Model;
use Magento\Framework\Pricing\PriceCurrencyInterface;
use oodles\ProductRE\Api\GetCustomProductListManagementInterface;
use Magento\Framework\App\RequestInterface;
use Magento\Catalog\Model\Product;
use phpDocumentor\Reflection\DocBlock\Tags\Var_;
class GetCustomProductListManagement implements GetCustomProductListManagementInterface
{
protected $productCollectionFactory;
protected $request;
protected $product;
protected $caloriemanagemt;
protected $_orderItems;
protected $orderItemRepo;
protected $itemFactory;
public function __construct(
\Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productCollectionFactory,
RequestInterface $request,
Product $product,
\Magento\Sales\Model\ResourceModel\Order\Item\CollectionFactory $orderItems,
\Magento\Sales\Model\Order\ItemFactory $itemFactory,
\Magento\Sales\Api\OrderItemRepositoryInterface $orderItem
){
$this->productCollectionFactory = $productCollectionFactory;
$this->request = $request;
$this->product = $product;
$this->_orderItems = $orderItems;
$this->itemFactory = $itemFactory;
$this->orderItemRepo = $orderItem;
}
/**
* GET for update order item Data api
* @api
* @return mixed|null
*/
public function updateOrderItem(){
//Get base64 order data (name,sku,price,order id,option's data)
$orderDatas = $this->request->getParam('order_data');
//Convert base64 data into array
$ordersData = json_decode(base64_decode($orderDatas),true);
foreach($ordersData as $orderData){
$order = $this->itemFactory->create()->getCollection()->addFieldToFilter('petdata', $orderData[0]["petdata"]);
foreach ($order as $items) {
$itemId = $items->getItemId();
$order = $this->orderItemRepo->get($itemId);
$option = $items->getProductOptions();
$option['info_buyRequest']['options'] = $orderData[0]['options'];//id of option
$option['options']['value'] = $orderData[0]['value']; //name of option
$option['options']['print_value'] = $orderData[0]['print_value']; // name of option
$order->setSku($orderData[0]['sku']);//set new sku
$order->setName($orderData[0]['name']);//set new name of product
$order->setPrice($orderData[0]['price']);// set updated price
$order->setProductOption($option);//set all option values
$order->save();
}
echo "success";
}
}
}

Step 3: - create web api file in app/code/oodles/ProductRE/etc/webapi.xml and paste below code 

<?xml version="1.0" ?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
<route url="/V1/oodles-productre/updateorderitem" method="POST">
<service class="oodles\ProductRE\Api\GetCustomProductListManagementInterface" method="updateOrderItem"/>
<resources>
<resource ref="anonymous"/>
</resources>
</route>
</routes>

Step 4: create di file in app/code/oodles/ProductRE/etc/di.xml and paste below code 

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="oodles\ProductRE\Api\GetCustomProductListManagementInterface" type="oodles\ProductRE\Model\GetCustomProductListManagement"/>
</config>

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