An implementation of Trees for PostgreSQL

tree.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.new attributes
      child.ancestry = ancestry + [id]
      child.save

      self.children = children + [child.id]
      save

      child
    end
  end

  def destroy
    unless children.empty?
      raise ActiveRecord::DeleteRestrictionError,
        "can not delete items with children"
    end

    transaction do
      parent.children = parent.children - [id]
      parent.save!

      super
    end
  end

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

  def parent
    @parent ||= self.class.find ancestry.last
  end
end