ancestry

https://github.com/stefankroes/ancestry

Ancestry allows the records of a ActiveRecord model to be organized in a tree structure, using the materialized path pattern. It exposes the standard relations (ancestors, parent, root, children, siblings, descendants) and allows them to be fetched in a single query. Additional features include named scopes, integrity checking, integrity restoration, arrangement of (sub)tree into hashes and different strategies for dealing with orphaned records.