If you have a list of ordered ids and you want to turn them into an ordered result set you can use
FIELD() in mysql:
SELECT * FROM addons
ORDER BY FIELD(id, 3, 5, 9, 1);
This is a handy trick if you use an external search engine which gives you an ordered list of ids and you want to pull out entire row sets.
We do this in addons.mozilla.org using the Django ORM like so:
The code in action.