export const metadata = {
  title: `${pageNumber} Common Data Model Definitions`,
}

# {metadata.title}

In this chapter, you'll learn where to find resources on common data model definitions related to field's column types, relations, and indices.

## MikroORM Learning Resources

Refer to the following resources to learn about common definitions:

1. [Field column types](https://mikro-orm.io/docs/defining-entities).
2. [Creating indices](https://mikro-orm.io/docs/defining-entities#indexes).

Refer to MikroORM's documentation for more details related to your use case.

---

## Relations Between Data Models

You can build relations between data models in the same module using foreign keys.

Refer to [MikroORM's relations definitions](https://mikro-orm.io/docs/relationships) for more details.

<Note>

For building relationships between data models in different modules, refer to the [Module Relationships chapter](../../modules/module-relationships/page.mdx) instead.

</Note>

---

## Example Data Model

The following example showcase a data model with common definitions:

```ts
import { 
  Entity, 
  Enum, 
  OneToOne, 
  PrimaryKey, 
  Property,
} from "@mikro-orm/core"
import { BaseEntity } from "@medusajs/utils"
// assuming this is another implemented data model
import ProductVariant from "./product-variant"

export enum MediaType {
  MAIN = "main",
  PREVIEW = "preview"
}

@Entity()
class ProductMedia extends BaseEntity {
  @PrimaryKey({ columnType: "text" })
  id: string

  @Property({ columnType: "text" })
  name: string

  @Enum({ items: ["main", "preview"] })
  type: MediaType

  @Property({ columnType: "text" })
  file_key: string
  
  @Property({ columnType: "text" })
  mime_type: string

  @Property({ columnType: "text" })
  variant_id: string

  @OneToOne({
    entity: ProductVariant,
    onDelete: "cascade",
  })
  variant: ProductVariant
}

export default ProductMedia
```

In the example above:

- The `ProductMedia` data model has the columns `id`, `name`, `type`, `file_key`, `mime_type`, and `variant_id`.
- The data model has an index on the `variant_id` column.
- The data model has a one-to-one relation to a `ProductVariant` data model.