Extension Attributes — Custom Attributes for Existing Entities

Defining Extension Attributes

Create an interface extending the entity's extension attributes interface:

namespace Vendor\Module\Api\Data;

use Magento\Catalog\Api\Data\ProductExtensionInterface;

interface ProductExtensionInterface extends ProductExtensionInterface
{
    const CUSTOM_FIELD = 'custom_field';
    const CUSTOM_SELECT = 'custom_select';
    
    public function getCustomField(): ?string;
    public function setCustomField(?string $value): self;
    public function getCustomSelect(): ?string;
    public function setCustomSelect(?string $value): self;
}

di.xml Configuration

Map the interface to your implementation:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Vendor\Module\Api\Data\ProductExtensionInterface"
                type="Vendor\Module\Model\Data\ProductExtension"/>
</config>

Implementation

namespace Vendor\Module\Model\Data;

use Magento\Framework\Api\ExtensibleDataObjectConverter;
use Vendor\Module\Api\Data\ProductExtensionInterface;

class ProductExtension implements ProductExtensionInterface
{
    private ?string $customField = null;
    private ?string $customSelect = null;
    
    public function getCustomField(): ?string {
        return $this->customField;
    }
    public function setCustomField(?string $value): self {
        $this->customField = $value;
        return $this;
    }
    public function getCustomSelect(): ?string {
        return $this->customSelect;
    }
    public function setCustomSelect(?string $value): self {
        $this->customSelect = $value;
        return $this;
    }
}

Database Table

Create a table for your extension attributes:

<!-- db_schema.xml -->
<table name="vendor_module_product_extension" resource="default" engine="innodb">
    <column xsi:type="int" name="id" padding="10" unsigned="true" nullable="false" identity="true"/>
    <column xsi:type="int" name="product_id" padding="10" unsigned="true" nullable="false"/>
    <column xsi:type="varchar" name="custom_field" nullable="true" length="255"/>
    <column xsi:type="varchar" name="custom_select" nullable="true" length="50"/>
    <constraint xsi:type="primary" referenceId="PRIMARY">
        <column name="id"/>
    </constraint>
    <constraint xsi:type="unique" referenceId="VENDOR_MODULE_PRODUCT_EXTENSION_PRODUCT_ID">
        <column name="product_id"/>
    </constraint>
    <constraint xsi:type="foreign" referenceId="FK_PRODUCT_ID">
        <column name="product_id"/>
        <referenceTable name="catalog_product_entity" referenceColumn="entity_id"/>
    </constraint>
</table>

Plugin to Load Extension Attributes

namespace Vendor\Module\Plugin;

use Magento\Catalog\Api\ProductRepositoryInterface;
use Vendor\Module\Api\Data\ProductExtensionInterfaceFactory;
use Vendor\Module\Model\ResourceModel\ProductExtension as ExtensionResource;

class ProductRepositoryPlugin
{
    public function __construct(
        private ProductExtensionInterfaceFactory $extensionFactory,
        private ExtensionResource $extensionResource
    ) {}

    public function afterGet(
        ProductRepositoryInterface $subject,
        \Magento\Catalog\Model\Product $product
    ) {
        $extension = $product->getExtensionAttributes();
        if (!$extension) {
            $extension = $this->extensionFactory->create();
        }
        $data = $this->extensionResource->load($product->getId());
        $extension->setCustomField($data['custom_field'] ?? null);
        $extension->setCustomSelect($data['custom_select'] ?? null);
        $product->setExtensionAttributes($extension);
        return $product;
    }
}

Plugin to Save Extension Attributes

namespace Vendor\Module\Plugin;

use Magento\Catalog\Api\ProductRepositoryInterface;
use Vendor\Module\Model\ResourceModel\ProductExtension as ExtensionResource;

class ProductRepositorySavePlugin
{
    public function __construct(
        private ExtensionResource $extensionResource
    ) {}

    public function beforeSave(
        ProductRepositoryInterface $subject,
        \Magento\Catalog\Model\Product $product,
        bool $saveOptions = false
    ) {
        $extension = $product->getExtensionAttributes();
        if ($extension) {
            $this->extensionResource->save($product->getId(), [
                'custom_field' => $extension->getCustomField(),
                'custom_select' => $extension->getCustomSelect()
            ]);
        }
    }
}

REST API Usage

// GET /rest/V1/products/SKU
{
    "sku": "SKU",
    "name": "Product Name",
    "extension_attributes": {
        "custom_field": "value",
        "custom_select": "option1"
    }
}

// PUT /rest/V1/products/SKU
{
    "product": {
        "sku": "SKU",
        "extension_attributes": {
            "custom_field": "new value"
        }
    }
}

GraphQL Usage

query {
  products(filter: { sku: { eq: "SKU" } }) {
    items {
      sku
      name
      custom_field
    }
  }
}

Naming Convention

// Prefix with your vendor/module
interface ProductExtensionInterface {
    const VENDOR_FIELD = 'vendor_field';
    
    public function getVendorField(): ?string;
    public function setVendorField(?string $value): self;
}

Null Safety

Always handle null values:

$extension = $product->getExtensionAttributes();
if ($extension && $extension->getCustomField() !== null) {
    // Process value
}

Entity Support

Extension attributes work for these entities:

Performance Considerations

• Load extension attributes only when needed (lazy loading)
• Use db_schema.xml for declarative schema
• Index frequently queried extension attribute columns
• Avoid N+1 queries when listing entities with extension attributes