# Documents, Collections and Subcollections — Firebase

Source: https://www.skillbyai.com/en/firebase/fs-docs

> The hierarchical data model.

## Paths alternate collection and document

Firestore data lives in **documents**: maps of fields (strings, numbers, booleans, timestamps, geopoints, references, arrays and nested maps). Documents live in **collections**, and a document can own **subcollections**, so paths alternate: `users/alice/orders/o1`. Collections are created implicitly when you write the first document, and deleting a document does **not** delete its subcollections. Documents have a size limit (1 MiB at the time of writing, check the docs), and a query on a collection returns documents from that collection only; **collection group** queries search every collection with the same ID, such as all `orders` subcollections.

## Documents in collections

Firestore stores JSON-like documents in collections, and you design the shape around the queries your screens need.

![Three ideas: documents and collections, modelling for queries, reads and writes.](assets/figures/firebase/section-3-map.svg) — Figure 3.1 — Collections, subcollections and modelling for reads.

## References to documents and collections

TypeScript.

```typescript
import { getFirestore, doc, collection, collectionGroup } from "firebase/firestore";

const db = getFirestore(app);

const userRef = doc(db, "users", "alice");                 // users/alice
const ordersCol = collection(db, "users", "alice", "orders"); // users/alice/orders
const orderRef = doc(ordersCol, "o1");                       // users/alice/orders/o1
const newOrderRef = doc(ordersCol);                          // auto-generated id

// every collection named "orders", at any depth
const allOrders = collectionGroup(db, "orders");
```

## Folders and files

Collections are folders, documents are files with fields, and a file can have its own folders inside it; deleting a file does not empty those folders automatically.

**Quiz:** What happens to a document's subcollections when the document is deleted from the client?

- [x] They remain and must be deleted separately
- [ ] They are deleted automatically
- [ ] They move to the parent collection
- [ ] The delete is rejected

*Answer:* They remain and must be deleted separately. Subcollections are independent of the parent document's existence.