This acts_as extension provides Nested Set functionality. Nested Set is similiar to Tree, but with the added feature that you can select the children and all of their descendents with a single query. A good use case for this is a threaded post system, where you want to display every reply to a comment without multiple selects.
class Product < ActiveRecord::Base acts_as_nested_set end
Copyright © 2007 David Heinemeier Hansson, released under the MIT license