What would be the best way to get next and previous record?

Let's say I have a class Post and I do a simple find for a record, like Post.find(16). Given this AR instance what would be the best way to get the previous and next records? I can't just use ids (15, 17 in this example) because records can be deleted.

Am I asking AR to do too much here and craft some custom SQL to handle this?

Thanks for the help.

found it!