Positioned module

positioned.rb

# Example:
#
# class Page < ActiveRecord::Base
#  include Positioned
# end

module Positioned
  def self.included model_class
    model_class.class_eval do
      before_save :set_position
    end
  end

  protected
  def set_position
    self.position ||= 1 + eval("#{self.class.to_s}.maximum(:position)").to_i
  end
end

Now available as a gem: https://github.com/sekrett/simple_position