We're looking into replacements for awesome_nested_set in ChiliProject mainly due to transactional safety concerns and the cost of one update/insert causing multiple writes on large trees, there's 1 thing we'd be missing though, and that's non-primary-key ancestry keys. The use-case would be for wiki pages and using the name of the page as key, so that I can get page nesting based on the path of the wiki page for free.
Example: I have /wiki/some-page and /wiki/some-page/subpage, and I'd get the correct nesting "for free", even better: if I first create /wiki/some-page/subpage and then /wiki/some-page, I'd still get nesting for free.
I'm not sure if we wouldn't need some form of scoping too (all wiki pages live in the same table but belong to different wikis), but I guess that would be easy to solve through named_scopes or similar.
Anyway, would you consider non-primary-key ancestry keys?