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.

<Note title="Use module links when" type="success">

- You want to create a relation between data models from different modules.

</Note>

<Note title="Don't use module links if" type="error">

- You want to create a relationship between data models in the same module. Use data model relationships instead.

</Note>

---

## 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(
  ProductModule.linkable.product,
  HelloModule.linkable.myCustom
)
```

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. Sync Links

Medusa stores links as pivot tables in the database. So, to reflect your link in the database, run the `links sync` command:

```bash
npx medusa links sync
```

Use this command whenever you make changes to your links. For example, run this command if you remove your link definition file.

---

## 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(
  ProductModule.linkable.product,
  {
    linkable: HelloModule.linkable.myCustom,
    isList: true,
  }
)
```

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.

---

## Set Delete Cascades on Link

To enable delete cascade on a link so that when a record is deleted, its linked records are also deleted, pass the `deleteCascades` property in the object passed to `defineLink`.

For example:

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

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

In this example, when a product is deleted, its linked `myCustom` record is also deleted.