Preorder traversal sequence on descendants

[elhoyos]

I believe this can be a very particular feature, but do you have any thoughts on making tag.descendants return its results with a preorder sequence? What would you recommend?

grandparent = Tag.find_or_create_by_path(["grandparent", "parentA", "childA"]).parent.parent
Tag.find_or_create_by_path(["grandparent", "parentB", "childB"])
grandparent.descendants
=> ["parentA", "parentB", "childA", "childB"] # current traversal sequence
=> ["parentA", "childA", "parentB", "childB"] # preorder traversal sequence 

[mceachen]

OK—how do you think the interface should look?

A) We add a new .preordered scope (so it'd be .descendants.preordered), or
B) Add a new specific-to-descendants scope, like preordered_descendants.

I think A) might be very difficult to apply generically (like, how would it work with ancestors?), so I'm leaning towards B).

For reference, there's http://en.wikipedia.org/wiki/Tree_traversal#Pre-order

[mceachen]

FWIW, preorder is exactly what this user is asking for over email:

I'm using a line like this with will paginate: @page.comments.paginate(page: 1, per_page: 4).hash_tree. With 5 test comments, one of which is a child of the first comment, this gives me:

first comment.
second comment.
third comment.
fourth comment.

But what I want is this:

first comment.
child of first comment.
second comment.
third comment.

[mceachen]

This is implemented in 3.8.0 as self_and_descendants_preordered: https://github.com/mceachen/closure_tree#deterministic-ordering

[leonelgalan]

Does a roots_and_descendants_preordered makes sense? I'm trying to show the whole tree (with multiple roots) in an outline preordered.

[mceachen]

@leonelgalan: Interesting, I'll think about it—it should be doable.

[mceachen]

@leonelgalan — just released 3.10.0, which has this new class method, roots_and_descendants_preordered. The README was updated as well.

Thanks for the suggestion!