Reverse orderByChild, store negative timestamp?

[David Notik]

I have orderByChild('createdDate') where createdDate is a millisecondsSinceEpoch timestamp.

The oldest users are coming out on top as expected.

But I'd like to sort newest on top. Does that mean I have to save a negative millisecondsSinceEpoch to a separate field? If so, may as well just use priorities, as they come w/ the added benefit that it won't show/pollute in Forge or in the JSON I receive. But is using priorities discouraged w/ the newer stuff?

Guidance appreciated. Thank you!

[David Notik]

I should note, the solution in this related post (to reverse the received results) doesn't apply to me:

https://groups.google.com/d/msg/firebase-talk/CRPUkk-FusQ/PEeBsrV-Y6YJ

That's because I'm not doing a .once('value') but a onChildAdded. In fact, multiple onChildAdded listeners as I infinite scroll down the page. It's all based on a query like:

var queryRef = f.child('/users3')
 .orderByChild('createdDate')
 .startAt(priority: lastPriority)
 .limitToFirst(pageSize + 1);

[Frank van Puffelen]

Hi David,

With child_added you get passed two events to the callback:

1. the DataSnapshot passed into the callback will reflect the data for the relevant child. 
2. For ordering purposes, it is passed a second argument which is a string containing the name of the previous sibling child
   

With this information you can keep the local items in (reverse) order, even when using child_added (and the other child_ events) instead of value. You just have to use limitToLast and mentally relabel previousChild to nextChild.

But there is also nothing wrong with adding an extra property for the reversed order. It's probably the quicker of the two solutions to implement.

       Frank

[Michael Lehenbauer]

Frank is right on.  Additionally:

1. Our leaderboard example (https://github.com/firebase/examples/blob/master/leaderboard/) uses this technique to show scores in descending order.  Might be useful.
2. We are starting to discourage the use of priorities, and new features in the future might not work with priorities.  So I'd use a regular field if you end up going that route.
3. This is a very reasonable request and we're looking at adding a way to "flip" the ordering client-side in the future to make it easier to render the results in the order of your choosing.

-Michael

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[David Notik]

Thank you both. It's good to have full confirmation of priorities going away – I recall Jacob had mentioned this months ago too.

I may just use the "second field" approach, but I also want to fully understand the limitToLast approach.

Do you mean e.g. https://github.com/firebase/examples/blob/master/leaderboard/js/leaderboard.js#L39?

It seems if I limitToLast(20) I'll get the very last 20 of whatever list as determined by orderBy_. If there are null values, those would be last in this case.

But do I also need endAt? I can't quite get this to work beyond the first page:

var queryRef = f.child('/users')


 .orderByChild('createdDate')
 .startAt(priority: lastPriority)


 .limitToLast(pageSize + 1);
...

queryRef.onChildAdded.listen((e) {
 var user = e.snapshot.val();
 lastPriority = user['createdDate'];
...


[David Notik]

And re: the "second field" approach, I called it "_priority" – any objections? :)

[Michael Lehenbauer]

Yeah, you'll need to use endAt as well.  Basically you'll use limitToLast/endAt instead of limitToFirst/startAt.  And instead of "lastPriority", you probably want "firstPriority."  Basically everything flips. :-)

Regarding null values, null values actually come first, before all non-null values.  So they'd probably show up in limitToFirst(10) rather than limitToLast(10).

-Michael

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