Counting amount of tracks without fetching all

[oskarrough]

In emberfire v2 (the version we're using) you can count the amount of tracks in a channel by checking the length of the ids in channels.tracks without fetching all tracks.

This is not possible in v3, see FirebaseExtended/emberfire#587.

If you're not using Emberfire, which is more and more likely, you can fetch a channel and count the tracks.length yourself. But having a number property like tracksCount would be very useful.

To do this we'll need to implement a Firebase cloud function. Copy/pasta from here https://github.com/firebase/functions-samples/tree/master/child-count

@hugurp should we do this? I'm not a huge fan of maintaining an extra cloud function, however small it is. On the other side, I want to be able to count tracks easily without fetching all tracks across our applications.

[oskarrough]

Having this properly would also help internet4000/radio4000#354 forward, since we can replace all the totalTracks stuff inside Radio4000.