Firebase should server or client do the filtering

Question

I am writing a polling app with a Firebase backend. Users answer to yes/no questions and see results. They can't see who answered the question though, only their friends can.

questions
    question1:
        readable
           text: "Do you enjoy Stack Overflow?"
           yes_count: 76
           no_count: 14
        response:
            user1: yes
            user2: no
            ...
            user1000: no

users:
    user1:
        friends:
            user1
            user999

One way to do this is to call onChildAdded on the response node and do filtering on the client side. This may result in downloading thousands of unnecessary UIDs.

Another way is to ask the server to do it. Users put requests to get friends' UIDs on a queue and the backend does the filtering and puts the result at a location that users can listen to. But this is a big extra load, especially considering that whenever there is a new answer to a question the backend has to check for every user whether that new answer is from a friend.

How do I query Firebase to get only friends' UIDs, instead of everyone's, at the response node?

If this is not possible, is it better for users to do filtering (and downloading of thousands of unneeded UIDs), or for the server to do it (huge extra load)?

Answer 1

This becomes trivial once you require that friendships are bi-directional, so also store the reverse relationship. E.g.

friends:
    user1:
        user42
        user999
    user42:
        user1
    user999:
        user1

With this bidirectional data structure, if a user posts you can simply load their friend list with /friends/$uid and loop over that. No expensive query needed.