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)