catalog_attributes.xml

Experiments

We use quote_item as an example.

We create a custom product attribute with product_brand code.

$productResp = $this->_objectManager->get('Magento\Catalog\Api\ProductRepositoryInterface');
$product = $productResp->getById(1);
var_dump($product->getData());

Ok, we can get product_brand attribute value.

$checkoutSession = $this->_objectManager->get('\Magento\Checkout\Model\Session');
$quote = $checkoutSession->getQuote();
$quoteItems = $quote->getAllItems();
foreach ($quoteItems as $item) {
    $product = $item->getProduct();
}
var_dump($product->getData());

However, we can’t get product_brand attribute value through qutoe item.

Let’s create a etc/catalog_attributes.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Catalog:etc/catalog_attributes.xsd">
    <group name="quote_item">
        <attribute name="product_brand"/>
    </group>
    <group name="wishlist_item">
        <attribute name="product_brand" />
    </group>
</config>

This time, we can get product_brand attribute value through qutoe item.

Source: vendor\magento\module-quote\Model\ResourceModel\Quote\Item\Collection.php::_assignProducts

...
$productCollection = $this->_productCollectionFactory->create()->setStoreId(
            $this->getStoreId()
        )->addIdFilter(
            $this->_productIds
        )->addAttributeToSelect(
            $this->_quoteConfig->getProductAttributes()
        );
...

$this->_quoteConfig is an instance of Magento\Quote\Model\Quote\Config

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Quote\Model\Quote;

class Config
{
    /**
     * @var \Magento\Catalog\Model\Attribute\Config
     */
    private $_attributeConfig;

    /**
     * @param \Magento\Catalog\Model\Attribute\Config $attributeConfig
     */
    public function __construct(\Magento\Catalog\Model\Attribute\Config $attributeConfig)
    {
        $this->_attributeConfig = $attributeConfig;
    }

    /**
     * @return array
     */
    public function getProductAttributes()
    {
        return $this->_attributeConfig->getAttributeNames('quote_item');
    }
}

Default Groups：
quote_item
wishlist_item
catalog_product
catalog_category
unassignable
used_in_autogeneration

References

How to access custom catalog attributes in Magento 2