Adding extension attribute to category api response

Question

The default response from magento 2 category api endpoint /rest/V1/categories/3 is

{
"id": 3,
"parent_id": 2,
"name": "Gear",
"is_active": true,
"position": 4,
"level": 2,
"children": "4,5,6",
"created_at": "2018-08-10 08:32:49",
"updated_at": "2018-08-24 08:02:27",
"path": "1/2/3",
"available_sort_by": [],
"include_in_menu": true,
"custom_attributes": [
    {
        "attribute_code": "description",
        "value": "<p>Gear Category Static Contents.</p>"
    },
    {
        "attribute_code": "display_mode",
        "value": "PAGE"
    },
    {
        "attribute_code": "landing_page",
        "value": "11"
    },
    {
        "attribute_code": "is_anchor",
        "value": "0"
    },
    {
        "attribute_code": "path",
        "value": "1/2/3"
    },
    {
        "attribute_code": "custom_layout_update",
        "value": "<referenceContainer name=\"catalog.leftnav\" remove=\"true\"/>"
    },
    {
        "attribute_code": "children_count",
        "value": "3"
    },
    {
        "attribute_code": "custom_use_parent_settings",
        "value": "0"
    },
    {
        "attribute_code": "custom_apply_to_products",
        "value": "0"
    },
    {
        "attribute_code": "url_key",
        "value": "gear"
    },
    {
        "attribute_code": "url_path",
        "value": "gear"
    }
]}

I would like to add a custom value to be passed in response, which is not derived from database, but generated based on certain calculation.

So my response should also contain

{
...
   {
    "attribute_code": "my_custom_flag",
    "value": "1"
   }
...
}

I tried the plugin approach and setting custom data in "afterGet" method, however it never shows up in response.

Appreciate any help on how to achieve this.

UPDATE: Basis Sairam's suggestion ...

Added di.xml as

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">    
<type name="Magento\Catalog\Api\CategoryRepositoryInterface">
    <plugin name="my_category_plugin" type="My\Catalog\Plugin\CategoryPlugin"/>
</type>

Added extension_attributes.xml as

<?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\Catalog\Api\Data\CategoryInterface">
    <attribute code="my_custom_flag" type="string"/>
</extension_attributes>

And the plugin file

<?php
namespace My\Catalog\Plugin;

use Magento\Catalog\Api\Data\CategoryExtensionFactory;
use Magento\Catalog\Api\Data\CategoryExtensionInterface;
use Magento\Catalog\Api\Data\CategoryInterface;
use Magento\Catalog\Api\CategoryRepositoryInterface;

class CategoryPlugin
{

protected $extensionFactory;

/**
 * @param \Magento\Catalog\Api\Data\CategoryExtensionFactory 
 * $extensionFactory
 */
public function __construct(\Magento\Catalog\Api\Data\CategoryExtensionFactory $extensionFactory)
{
    $this->extensionFactory = $extensionFactory;
}

/**
 * @param CategoryRepositoryInterface $subject
 * @param CaetgoryInterface $category
 *
 * @return CaetgoryInterface
 */
public function afterGet(\Magento\Catalog\Api\CategoryRepositoryInterface $subject, \Magento\Catalog\Api\Data\CategoryInterface $category)
{
    $extensionAttributes = $category->getExtensionAttributes();
    $extensionAttributes = $extensionAttributes ? $extensionAttributes : $this->extensionFactory->create();

    // set custom flag
    $extensionAttributes->setMyCustomFlag('1');

    // $extensionAttributes->setExtensionAttribute('my_custom_flag', '1');

    $category->setExtensionAttributes($extensionAttributes);

    return $category;
 }
}

However, the response from the API call never changes, it's still missing "my_custom_flag". It remains unaffected.

Answer 1

Create a extension attribute for the required datainterface here I'm providing sample:

<extension_attributes for="Magento\Store\Api\Data\StoreConfigInterface">
    <attribute code="sample" type="string"/>
</extension_attributes>

Have a plugin on the interface di.xml

<type name="Magento\Store\Api\StoreConfigManagerInterface">
    <plugin name="nameofplugin"
            type="Namespace\Modulename\Plugin\filename"/>
</type>

Plugin class

    <?php

      namespace Emc\App\Plugin;

      use Magento\Store\Api\Data\StoreConfigExtensionInterface;
      use Magento\Store\Api\Data\StoreConfigInterface;
      use Magento\Store\Api\Data\StoreConfigExtensionFactory;
      use Magento\Setup\Exception;
      use Magento\Directory\Model\Currency;
      use Magento\Store\Model\StoreManagerInterface;


/**
 * Class classname
 */
class classname
{
    protected $storeConfigExtensionFactory;
    protected $storeConfigInterface;
    protected $storeConfigExtensionInterface;
    protected $scopeConfig;

    /**
     * Product constructor.
     * @param \Magento\Catalog\Model\ProductRepository $productRepository
     */
    // objects need to be changed based on your requirement 
    public function __construct(
    Currency $currencyFactory, 
    StoreManagerInterface $storeManager, 
    StoreConfigExtensionFactory $storeConfigExtensionFactory, 
    StoreConfigInterface $storeConfigInterface, 
    \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
    ) {
            $this->storeConfigExtensionFactory = $storeConfigExtensionFactory;
            $this->storeConfigInterface = $storeConfigInterface;
            $this->scopeConfig = $scopeConfig;
            $this->currencyFactory = $currencyFactory;
            $this->storeManager = $storeManager;

     }


    /**
     * @param string[] $storeCodes list of stores by store codes, will return all if storeCodes is not set
     * @return \Magento\Store\Api\Data\StoreConfigInterface[]
     */
    public function aroundGetStoreConfigs(\Magento\Store\Model\Service\StoreConfigManager $storeConfigManager, \Closure $proceed, array $storeCodes = null)
    {

        $result = $proceed($storeCodes);
        foreach ($result as $storevalues) {
            $storeconfigExtension = $this->storeConfigExtensionFactory->create();
->getCurrencySymbol());
            $storeconfigExtension->setSample($storedKey); //attribute code

            $storevalues->setExtensionAttributes($storeconfigExtension);
        }
        return $result;
    }

}

Answer 2

I don't see any issue on your extension attribute code. Maybe you just need to make sure the source code have been upload to your Magento server correctly, then try to clear the cache / run the di compile command if needed.