gistfile1.rb

module Tree
  extend ActiveSupport::Concern

  module ClassMethods
    def leaves
      where("children = ?::integer[]", [].pg)
    end

    def roots
      where("ancestry = ?::integer[]", [].pg)
    end
  end

  def add_child(attributes)
    transaction do
      child = self.class.create attributes.merge(ancestry: ancestry + [id])
      update_attributes(children: children + [child.id])

      child
    end
  end

  def parents
    self.class.where(id: ancestry)
  end

  def parent
    self.class.find ancestry.last
  end
end