extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<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\ProductInterface">
        <!--
        @deprecated Stock Item as part of ExtensionAttributes is deprecated
        @see StockItemInterface Should be used when you would like to "write" data into the data storage
        @see StockStatusInterface It should be used to "read" data for representation (on front-end)
        @see StockItemRepositoryInterface::save as extension point for logic customization
        -->
        <attribute code="stock_item" type="Magento\CatalogInventory\Api\Data\StockItemInterface">
            <resources>
                <resource ref="Magento_CatalogInventory::cataloginventory"/>
            </resources>
        </attribute>
    </extension_attributes>
</config>