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):

{
  "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.

db.collection('landmarks/golden_gate_bridge/reviews').where('rating', '==', 5)

Collection Group Query: Find all 5-star reviews across all landmarks.

db.collectionGroup('reviews').where('rating', '==', 5)

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