In Google Cloud Firestore, many-to-many relationships can be modeled using intermediary collections. For instance, if we have two entities ‘Users’ and ‘Groups’, a user can belong to multiple groups and vice versa. To model this relationship, an intermediary collection ‘UserGroups’ is created. Each document in ‘UserGroups’ contains the IDs of both a User and a Group, representing membership. This allows for efficient querying of all groups a user belongs to or all users in a group by querying the ‘UserGroups’ collection with the relevant ID.
