How to set firebase-database rule so users can read each other's data iff they share a group

Question

I have a chat-like realtime database with the following design

groups-
       |
       [groupId1]-
                 |
                 users-
                       |
                        [uid]
                        ....
       [groupId2]-
                 |
                 users-
                       |
                        [uid]
                        ....

users-
       |
       ....
       [uid]-
             |
             groups-
                   |
                   [groupId1]
                   [groupId2]

I want to set up Rules so user1 can ".read" data from users/user2 iff they share at least one group. The data structure is such that i need every user can be in numerous groups so the users data must be under users/uid node and cannot be duplicated to every groups/[groupId]/users/[uid]

Any ideas?

Answer 1

You can find the answer to your question here with the Chat Example : https://firebase.google.com/docs/database/security/user-security#section-revisiting-advanced-example