atomic - How do I prevent duplicate voting while incrementing the vote count on Firebase?

Question

I'm building a real-time poll in Firebase. Each vote is stored in a list field. To prevent having to pull every vote down to the client in order to count them, I cache the tallies for each choice in counter fields.

poll1
    counts
        choice1: 5
        choice2: 2
        choice3: 10
        choice4: 252
    voters
        uid1 : choice1
        uid6 : choice3
        uid25: choice2
        uid31: choice1

I'm currently updating the counter with the following transaction:

var fireCountPush = new Firebase(self.fireCountUrl+node+id);
fireCountPush.transaction(function(current_value) {
    return current_value + 1;
}, function(error, committed, snapshot) {
    if(committed) {
        var fireVote = new Firebase(self.fireVotesUrl);
        fireVote.child(self.user.uid).set(id);
    }
});

However, I want to atomically add the user to the voters list, ideally in the same transaction. Unfortunately, the way I have it now, I'm having to add the user after the transaction commits successfully. This is a huge security issue since it can be easily disabled in the browser by editing the script.

Is there any way to both update the counter and add a user to the list of voters without having to download the entire object in the transaction?

Answer 1

Waitting for answers