Realtime Database

Data is saved and queried in a tree structure.

When you:

• fetch data at a location in your database, you also retrieve all of its child nodes.
• grant someone read or write access at a node in your database, you also grant them access to all data under that node

For a two-way relationship, the relationship needs to be duplicated (ie saved twice). The relationship is saved:

• in the user's record
• and under the group.

So to delete a user from the group, it has to be updated in two places. This is a necessary redundancy for two-way relationships in order to scale.

doc

Cloud Firestore

Cloud Firestore is a document database.