---
description: 'Learn how to extend a core entity in Medusa to add custom attributes.'
addHowToData: true
---

# How to Extend an Entity

In this document, you’ll learn how to extend a core entity in Medusa.

## Overview

Medusa uses entities to represent tables in the database. As you build your custom commerce application, you’ll often need to add your own properties to those entities. This guide explains the necessary steps to extend core Medusa entities.

This guide will use the Product entity as an example to demonstrate the steps.

### Word of Caution about Overriding

Extending entities to add new attributes or methods shouldn't cause any issues within your commerce application. However, if you extend them to override their existing methods or attributes, you should be aware that this could have negative implications, such as unanticipated bugs, especially when you try to upgrade the core Medusa package to a newer version.

---

## Step 1: Create Entity File

In your Medusa backend, create the file `src/models/product.ts`. This file will hold your extended entity.

Note that the name of the file must be the same as the name of the original entity in the core package. Since in this guide you’re overriding the Product entity, it’s named `product` to match the core. If you’re extending the customer entity, for example, the file should be named `customer.ts`.

---

## Step 2: Implement Extended Entity

In the file you created, you can import the entity you’re extending from the core package, then create a class that extends that entity. You can add in that class the new attributes and methods.

Here’s an example of extending the Product entity:

```ts title=src/models/product.ts
import { Column, Entity } from "typeorm"
import {
  // alias the core entity to not cause a naming conflict
  Product as MedusaProduct,
} from "@medusajs/medusa"

@Entity()
export class Product extends MedusaProduct {
  @Column()
  customAttribute: string
}
```

---

## (Optional) Step 3: Create a TypeScript Declaration File

If you’re using JavaScript instead of TypeScript in your implementation, you can skip this step.

To ensure that TypeScript is aware of your extended entity and affects the typing of the Medusa package itself, create the file `src/index.d.ts` with the following content:

```ts title=src/index.d.ts
export declare module "@medusajs/medusa/dist/models/product" {
  declare interface Product {
    customAttribute: string;
  }
}
```

Notice that you must pass the attributes you added to the entity into the `interface`. The attributes will be merged with the attributes defined in the core `Product` entity.

---

## Step 4: Create Migration

To reflect your entity changes on the database schema, you must create a migration with those changes.

You can learn how to create or generate a migration in [this documentation](./migrations/create.md).

Here’s an example of a migration of the entity extended in this guide:

```ts title=src/migration/1680013376180-changeProduct.ts
import { MigrationInterface, QueryRunner } from "typeorm"

class changeProduct1680013376180 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(
      "ALTER TABLE \"product\"" + 
      " ADD COLUMN \"customAttribute\" text"
    )
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(
      "ALTER TABLE \"product\" DROP COLUMN \"customAttribute\""
    )
  }
}

export default changeProduct1680013376180
```

---

## Step 5: Use Custom Entity

For changes to take effect, you must transpile your code by running the `build` command in the root of the Medusa backend:

```bash npm2yarn
npm run build
```

Then, run the following command to migrate your changes to the database:

```bash
npx medusa migrations run
```

You should see that your migration was executed, which means your changes were reflected in the database schema.

You can now use your extended entity throughout your commerce application.

---

## Access Custom Attributes and Relations in Core Endpoints

### Request Parameters

In most cases, after you extend an entity to add new attributes, you'll likely need to pass these attributes to endpoints defined in the core. By default, this causes an error, as request parameters are validated to ensure only those that are defined are passed to the endpoint.

To allow passing your custom attribute, you'll need to [extend the validator](../endpoints/extend-validator.md) of the endpoint.

### Response Fields

After you add custom attributes, you'll notice that these attributes aren't returned as part of the response fields of core endpoints. Core endpoints have a defined set of fields and relations that can be returned by default in requests.

To change that and ensure your custom attribute is returned in your request, you can extend the allowed fields of a set of endpoints in a loader file and add your attribute into them.

For example, if you added a custom attribute in the `Product` entity and you want to ensure it's returned in all the product's store endpoints (endpoints under the prefix `/store/products`), you can create a file under the `src/loaders` directory in your Medusa backend with the following content:

```ts title=src/loaders/extend-product-fields.ts
export default async function () {
  const imports = (await import(
    "@medusajs/medusa/dist/api/routes/store/products/index"
  )) as any
  imports.allowedStoreProductsFields = [
    ...imports.allowedStoreProductsFields,
    "customAttribute",
  ]
  imports.defaultStoreProductsFields = [
    ...imports.defaultStoreProductsFields,
    "customAttribute",
  ]
}
```

In the code snippet above, you import `@medusajs/medusa/dist/api/routes/store/products/index`, which is where all the product's store endpoints are exported. In that file, there are the following defined variables:

- `allowedStoreProductsFields`: The fields or attributes of a product that are allowed to be retrieved and returned in the product's store endpoints. This would allow you to pass your custom attribute in the `fields` request parameter of the product's store endpoints.
- `defaultStoreProductsFields`: The fields or attributes of a product that are retrieved and returned by default in the product's store endpoints.

You change the values of these variables and pass the name of your custom attribute. Make sure to change `customAttribute` to the name of your custom attribute.

:::tip

Before you test out the above change, make sure to build your changes before you start the Medusa backend.

:::

You can also add custom relations by changing the following defined variables:

- `allowedStoreProductsRelations`: The relations of a product that are allowed to be retrieved and returned in the product's store endpoints. This would allow you to pass your custom relation in the `expand` request parameter of the product's store endpoints.
- `defaultStoreProductsRelations`: The relations of a product that are retrieved and returned by default in the product's store endpoints.

If you want to apply this example for a different entity or set of endpoints, you would need to change the import path `@medusajs/medusa/dist/api/routes/store/products/index` to the path of the endpoints you're targeting. You also need to change `allowedStoreProductsFields` and `defaultStoreProductsFields` to the names of the variables in that file, and the same goes for relations. Typically, these names would be of the format `(allowed|default)(Store|Admin)(Entity)(Fields|Relation)`.

---

## Advanced Entity Definitions

With entities, you can create relationships, index keys, and more. As Medusa uses Typeorm, you can learn about using these functionalities through [Typeorm's documentation](https://typeorm.io/).
