Managing unread messages in firebase chat

Question

I'm building real time chat, very similar with skype one. I use firebase as backend and angularfire on client side. Basically, all things are look clear, but I've stuck with one thing - showing of unread messages count.

App uses very simple Firebase design: 2 or more users can chat in a "room" - root collection with unique name. Chat message except of text can contain "metadata" - sender id, timestamp, etc.

Simply, I need an emulation of this pseudo-code:

room.messages.where({ unread: true }).count()

For now, according to this, (Can I count the children at a location without retrieving the actual child data?) and this I'm trying to manage unread messages count per room by transactions and reset count when viewed. But it is very tricky part, and I'm curious, do we have any recommended approach here, which can reduce amount of job?

Answer 1

It seems like you've pretty much answered it. There is no WHERE clause in Firebase, and the solution is to use snap.numChildren() as the FAQ states, or to use a counter, as the second link states.

If you are going to fetch the chat messages anyway, or it's a one-on-one chat where the total payload would be a hundred kilobytes or less (twenty or so 10kb messages), then just use numChildren. If the message payload is going to be rather large, then set up the counters.

So you would maintain a counter of:

• how many messages your user has read
• how many messages exist
• the difference is the number of unread messages

Since your "messages exist" counter would be updated by multiple users concurrently, you'd use a transaction to accomplish this without conflicts:

new Firebase(URL_TO_COUNTER).transaction(function(currValue) {
    return (currValue||0)+1;
}, function(err, success, snap) {
    if( err ) { throw err; }
    console.log('counter updated to '+snap.val());
});