# Structure your Makerkit Application | Next.js Firebase SaaS Kit

> Learn how to structure your application with additional entities and business logic in your Next.js Firebase SaaS Kit

*Canonical: https://makerkit.dev/docs/next-fire/architecture/application-structure*

---

In the previous section, we learned the fundamentals of Makerkit's architecture and the application layers.

In this section, we learn how to structure your application in practical terms with an example. For example, your application has an entity "events": how do we add this entity to a Makerkit application?

{% alert type="default" title="Entities are added to core" %}
NB: entities rarely (or never) get added to "core". Business domain is added to "components" and "lib".
{% /alert %}

In short, this is how we add a new entity to the application:

1. First, we add a new folder to `lib`. If the entity is "event", we add `lib/events`.
2. Then, we add the components of the `event` domain to `components/events`
3. Finally, we add the pages of the `event` domain to `pages/events`

 ```txt {% title="Structure" %}
- src
  - components
    - events
      - EventsContainerComponent.tsx
      - ...

  - lib
    - events
      - types
        - event-model.ts
        - ...
      - hooks
        - use-fetch-events.ts
        - use-create-event.ts
        - ...
      - utils
        - create-event-model.ts

  - pages
    - events
      - page.tsx
      - [event].tsx
```

### 1) Adding the entity's business domain

We will add various business logic units in the `lib/events` folder, such as types, custom hooks, API calls, factories, functions, utilities, etc.

#### Types

First, we define a type `EventModel` at `lib/events/types/event-model.ts`:

 ```tsx {% title="lib/events/types/event-model.ts" %}
export interface EventModel {
  name: string;
  description: string;
}
```

#### Custom Hooks

For example, let's write a custom hook that retrieves a list of "events" from a Firestore collection.

We create a file at `lib/events/hooks/use-fetch-events.ts` with the following content:

 ```tsx {% title="src/lib/events/hooks/use-fetch-events.ts" %}
import EventModel from '~/lib/events/types/event-model'

export function useFetchEvents() {
  const firestore = useFirestore();
  const eventsCollection = 'events';

  const collectionRef = collection(
    firestore,
    eventsCollection,
  ) as CollectionReference<EventModel>;

  return useFirestoreCollectionData(collectionRef, {
    idField: 'id',
  });
}
```

Good! We have a way to fetch our events, but we have to use it somewhere: to do so, let's create a component `EventsListContainer`. 

{% alert type="warning" title="Update Firestore rules" %}
NB: remember to update the Firestore rules to be able to read the collection.
{% /alert %}

### 2) Components

As said before, we add React components that belong to the "events" domain to `components/events`. 

In the component below, we will fetch a list of events with `useFetchEvents`:

 ```tsx {% title="components/events/EventsListContainer.tsx" %}
import { useFetchEvents } from '~/lib/events/hooks/use-fetch-events';
import { Alert } from `~/core/ui/Alert`;

const EventsListContainer: React.FC = () => {
  const { data: events, status } = useFetchEvents();

  if (status === `loading`) {
    return <p>Loading Events...</p>
  }

  if (status === `error`) {
    return (
      <Alert type='error'>
        Ops, we encountered an error!
      </Alert>
    );
  }

  return (
    <div>
      {events.map(event => {
        return (
          <div key={event.name}>
            <p>{event.name}</p>
            <p>{event.description}</p>;
          </div>
        );
      })}
    </div>
  )
};

export default EventsListContainer;
```

### 3) Pages

Finally, we can add the events component `EventsListContainer` to a page. To do so, let's create a page component at `pages/events/index.ts`:

 ```tsx {% title="pages/events/index.ts" %}
import { GetServerSidePropsContext } from 'next';

import RouteShell from '~/components/RouteShell';
import EventsListContainer from '~/components/EventsListContainer';

import { withAppProps } from '~/lib/props/with-app-props';

const EventsPage: React.FC = () => {
  return (
    <>
      <Head>
        <title key="title">Events</title>
      </Head>
      
      <RouteShell title="Events">
        <EventsListContainer />
      </RouteShell>
    </>
  );
};

export default EventsPage;

// guard the page!
export function getServerSideProps(ctx: GetServerSidePropsContext) {
  return withAppProps(ctx);
}
```

🎉 That's it! We have now built a nicely structured "events" domain.