export const metadata = {
  title: `${pageNumber} Module Link`,
}

# {metadata.title}

In this chapter, you’ll learn what a module link is.

<Note type="soon" title="In Development">

Module links are in active development.

</Note>

## What is a Module Link?

A module link forms an association between two data models of different modules, while maintaining module isolation.

You can then retrieve data across the linked modules, and manage their linked records.

---

## Prerequisite: isQueryable Configuration

Before you define a module link, you must enable the `isQueryable` configuration of the module.

For example:

```js
module.exports = defineConfig({
  // ...
  modules: {
    helloModuleService: {
      resolve: "./modules/hello",
      definition: {
        isQueryable: true,
      },
    },
  },
})
```

---

## How to Define a Module Link?

### 1. Create Link File

Links are defined in a TypeScript or JavaScript file under the `src/links` directory. The file defines the link using the `defineLink` function imported from `@medusajs/utils` and exports it.

For example:

export const highlights = [
  ["6", "linkable", "Special `linkable` property that holds the linkable data models of `HelloModule`."],
  ["7", "linkable", "Special `linkable` property that holds the linkable data models of `ProductModule`."],
]

```ts title="src/links/hello-product.ts" highlights={highlights}
import HelloModule from "../modules/hello"
import ProductModule from "@medusajs/product"
import { defineLink } from "@medusajs/utils"

export default defineLink(
  HelloModule.linkable.myCustom,
  ProductModule.linkable.product
)
```

The `defineLink` function accepts as parameters the link configurations of each module's data model. A module has a special `linkable` property that holds these configurations for its data models.

In this example, you define a module link between the `hello` module's `MyCustom` data model and the Product Module's `Product` data model.

### 2. Run Migrations

Medusa stores links as pivot tables in the database, so you must run migrations after defining a link:

```bash
npx medusa migrations run
```

---

## Define a List Link

By default, the defined link establishes a one-to-one relation: a record of a data model is linked to one record of the other data model.

To specify that a data model can have multiple of its records linked to the other data model's record, use the `isList` option.

For example:

```ts
import HelloModule from "../modules/hello"
import ProductModule from "@medusajs/product"
import { defineLink } from "@medusajs/utils"

export default defineLink(
  {
    linkable: HelloModule.linkable.myCustom,
    isList: true
  },
  ProductModule.linkable.product
)
```

In this case, you pass an object of configuration as a parameter instead. The object accepts the following properties:

- `linkable`: The data model's link configuration.
- `isList`: Whether multiple records can be linked to one record of the other data model.

In this example, a record of `product` can be linked to more than one record of `myCustom`.

---

## Extend Data Models with Module Links

Module links are most useful when you want to add properties to a data model of another module.

For example, to add custom properties to the `Product` data model of the Product Module, you:

1. Create a module.
2. Create in the module a data model that holds the custom properties you want to add to the `Product` data model.
2. Define a module link that links your module to the Product Module.

Then, in the next chapters, you'll learn how to:

- Link each product to a record of your data model.
- Retrieve your data model's properties when you retrieve products.