descendants.rb

# This module defines a class method called "descendants" that will return a
# list of all classes that inherit from the current class.
#
#   Array.descendants
#   Hash.descendants
#   ActiveRecord::Base.descendants
#
module Descendants
  def descendants
    ary = []
    ObjectSpace.each_object(self.class) {|obj| ary << obj if obj < self}
    return ary
  end
end

Class.class_eval 'include Descendants'