thing.rb

# assuming the following columns in `things`
# 
# children_counter_cache  :integer
# parent_id               :integer
#
class Thing < ActiveRecord::Base
  has_many :children, :class_name => "Thing", :foreign_key => "parent_id", :dependent => :nullify
  belongs_to :parent, :class_name => "Thing", :foreign_key => "parent_id", :counter_cache => 'children_counter_cache'  
end