How to add Extension Attributes for customers in Magento 2?

Magento provides two types of attributes for entities. And here we going to learn how to add a new extension attribute in Magento. Please follow the below steps to add the extension attribute:

Step 1: Add extension attribute

Here we will add an extension attribute in extension_attributes.xml which resides under the etc folder of your custom module.

For scalar attributes:

<?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\Customer\Api\Data\CustomerInterface">
        <attribute code="stripe_id" type="boolean" />
    </extension_attributes>
</config>

Here, the scalar attributes indicate the simple form of attribute representation, such as an integer, a string, or a boolean. Specify the class or interface of the extension attributes inside the “for” attribute of the <extension_attributes> tag. In this case, it is the CustomerInterface. The attribute is specified with a unique code and its type.

For non-scalar attributes:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <type name="Magento\Customer\Api\CustomerRepositoryInterface">
        <plugin name="add_stripe_id" type="Magento\SomeModule\Api\Data\CustomInterface"/>
    </type>
</config>

Here, the non-scalar attributes indicate data objects such as the instance of a class. In the above example, the CustomDataInterface object is added as an extension attribute.

Step 2: Add/Update extension attribute values using Plugin

To add an extension attribute with customer data we need to add a plugin for Customer Repository. A plugin should be created for individual methods which are used like: get, save, and getById. Here we will see how to pass additional data in customer sessions. For this follow below:

Add plugin in di.xml file:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <type name="Magento\Customer\Api\CustomerRepositoryInterface">
        <plugin name="add_stripe_id" type="Magento\SomeModule\Plugin\CustomerPlugin"/>
    </type>
</config>

Here I have mentioned the plugin file name as CustomerPlugin so create a file inside the Plugin folder with CustomerPlugin.php

<?php

namespace Magento\SomeModule\Plugin;

use Magento\Customer\Api\CustomerRepositoryInterface;
use Magento\Customer\Api\Data\CustomerInterface;
use Magento\Framework\Api\ExtensionAttributesFactory;
use Magento\Customer\Api\Data\CustomerExtensionInterface;
use Magento\Framework\App\ResourceConnection;

class CustomerPlugin
{

    /**
     * @var ExtensionAttributesFactory
     */
    private $extensionFactory;

    /**
     * @var ResourceConnection
     */
    private $resourceConnection;

    /**
     * @param ExtensionAttributesFactory $extensionFactory
     * @param ResourceConnection $resourceConnection
     */
    public function __construct(
        ExtensionAttributesFactory $extensionFactory,
        ResourceConnection $resourceConnection
    ) {
        $this->extensionFactory = $extensionFactory;
        $this->resourceConnection = $resourceConnection;
    }

    /**
     * Plugin after getById customer that adds customer's stripeId.
     *
     * @param CustomerRepositoryInterface $subject
     * @param CustomerInterface $customer
     * @return CustomerInterface
     */
    public function afterGetById(CustomerRepositoryInterface $subject, CustomerInterface $customer)

    {
        $extensionAttributes = $customer->getExtensionAttributes();
        
        if ($extensionAttributes === null || $extensionAttributes->getStripeId() === null) {
            $connection = $this->resourceConnection->getConnection();
            $select = $connection->select()->from(
                ['stripe_customers'], ['stripe_id'] 
            )->where("customer_id = ?", $customer->getId());
            $stripeId = $connection->fetchOne($select);
            $this->addStripeIdExtensionAttribute($customer, $stripeId);
        }
        return $customer;
    }

    /**
     * Set Stripe Id extension attribute
     *
     * @param CustomerInterface $customer
     * @param string $stripeId
     */
    private function addStripeIdExtensionAttribute(CustomerInterface $customer, string $stripeId): void
    {
        $extensionAttributes = $customer->getExtensionAttributes();
        if ($extensionAttributes === null) {
            /** @var CustomerExtensionInterface $extensionAttributes */
            $extensionAttributes = $this->extensionFactory->create(CustomerInterface::class);
            $customer->setExtensionAttributes($extensionAttributes);
        }
        $extensionAttributes->setStripeId($stripeId);
    }
}

Here we have added plugin for getById method only. Similiarly we can add plugin for other methods as well according to the need.