# Firestore Data Model Reference

Firestore is a NoSQL, document-oriented database. Unlike a SQL database, there are no tables or rows. Instead, you store data in **documents**, which are organized into **collections**.

## Document Data Model

Data in Firestore is organized into documents, collections, and subcollections.

### Documents
A **document** is a lightweight record that contains fields, which map to values. Each document is identified by a name. A document can contain complex nested objects in addition to basic data types like strings, numbers, and booleans. Documents are limited to a maximum size of 1 MiB.

Example document (e.g., in a `users` collection):
```json
{
  "first": "Ada",
  "last": "Lovelace",
  "born": 1815
}
```

### Collections
Documents live in **collections**, which are containers for your documents. For example, you could have a `users` collection to contain your various users, each represented by a document.
* Collections can only contain documents. They cannot directly contain raw fields with values, and they cannot contain other collections.
* Documents within a collection can contain different fields.
* You don't need to "create" or "delete" collections explicitly. After you create the first document in a collection, the collection exists. If you delete all of the documents in a collection, the collection no longer exists.

### Subcollections
Documents can contain subcollections natively. A subcollection is a collection associated with a specific document.
For example, a user document in the `users` collection could have a `messages` subcollection containing message documents exclusively for that user. This creates a powerful hierarchical data structure.

Data path example: `users/user1/messages/message1`

## Collection Group Support

A **collection group** consists of all collections with the same ID. By default, queries retrieve results from a single collection in your database. Use a collection group query to retrieve documents from a collection group instead of from a single collection.

### Use Cases
Collection group queries are useful when you want to query across multiple subcollections that share the same organizational structure.

For example, imagine an app with a `landmarks` collection where each landmark has a `reviews` subcollection. If you want to find all 5-star reviews across *all* landmarks, it would involve checking many separate `reviews` subcollections. With a collection group, you can perform a single query against the `reviews` collection group.

### Examples

**Standard Query** (Single Collection):
Find all 5-star reviews for a specific landmark.
```javascript
db.collection('landmarks/golden_gate_bridge/reviews').where('rating', '==', 5)
```

**Collection Group Query**:
Find all 5-star reviews across *all* landmarks.
```javascript
db.collectionGroup('reviews').where('rating', '==', 5)
```