# Add entities to domains

Copy as Markdown[View as Markdown](/docs/development/guides/domains/add-resources-to-domains/add-entities-to-domains.md)

***

Once you have [created your entities](/docs/development/guides/resources/entities/create-entity.md) you can add them to your domains.

To add an entity to a domain, add the entity to the `entities` array in the domain's markdown file.

/domains/Orders/index.mdx

```

---
id: Orders
name: Orders
version: 1.0.0
entities:
  - id: Order
    version: 1.0.0
  - id: OrderItem
    version: 1.0.0
---

Your domain markdown...
```

## Visualize domain entities[​](#visualize-domain-entities "Direct link to Visualize domain entities")

**Added in** `eventcatalog@2.49.0`

When a domain has entities assigned, EventCatalog can generate an entity map for that domain.

The entity map helps teams understand the domain model at a glance. It shows:

* The entities inside the domain
* The relationships between those entities
* Which entities are owned by the current domain
* Entities referenced from other domains

Entities that are referenced from another domain are shown as **yellow**.

![Entity Map](/assets/images/entity-map-e1404c5598cceb7732a548eae61bc0e6.png)

## View the entity map in the visualizer[​](#view-the-entity-map-in-the-visualizer "Direct link to View the entity map in the visualizer")

EventCatalog creates a dedicated entity map visualizer route for domains:

```
/visualiser/domains/{id}/{version}/entity-map
```

For services, the route is:

```
/visualiser/services/{id}/{version}/entity-map
```

When a domain or service has entities assigned, an **Entity Map** link appears automatically in the resource sidebar under the visualizer section.

## Embed the entity map on a page[​](#embed-the-entity-map-on-a-page "Direct link to Embed the entity map on a page")

You can also embed the entity map of a domain on any page in your catalog using the `<EntityMap />` component.

```
<EntityMap id="Orders" title="Orders Domain Entity Map" />
```

![Entity Map Component](/assets/images/entity-map-component-2fd5621f4528ad14c72e627e80d6a471.png)

## Create relationships between entities[​](#create-relationships-between-entities "Direct link to Create relationships between entities")

Entity maps become more useful when your entities reference each other.

Use the optional reference fields on entity properties:

* `references`: The entity you want to reference, for example `Order`
* `relationType`: The type of relationship, for example `hasOne`, `hasMany`, or `belongsTo`
* `referencesIdentifier`: The property in the referenced entity used to establish the relationship, for example `orderId`

For example, an `OrderItem` can reference an `Order` through `orderId`.

/domains/Orders/entities/OrderItem/index.mdx

```
---
id: OrderItem
name: OrderItem
version: 1.0.0
identifier: orderItemId
summary: Represents a single item within a customer's order.

properties:
  - name: orderItemId
    type: UUID
    required: true
    description: Unique identifier for the order item
  - name: orderId
    type: UUID
    required: true
    description: Identifier for the parent Order
    references: Order
    relationType: hasOne
    referencesIdentifier: orderId
---
```

This configuration generates an entity map that visually shows the relationship between `OrderItem` and `Order`.

![Entity Map showing a relationship between OrderItem and Order](/assets/images/simple-entity-map-ce7d68af60003de0a155881e19d9c3a9.png)

More information on entity reference fields can be found in the [entities reference](/docs/development/guides/resources/entities/reference.md).