rankable.rb

require 'active_support/concern'

module Rankable
  extend ActiveSupport::Concern
  included do
    validates :row_order, :presence => true
    scope :next_rank, lambda { |rank| where('row_order > ?',rank).order("row_order asc").limit(1)}
    scope :previous_rank, lambda { |rank| where('row_order < ?',rank).order("row_order desc").limit(1)}
    scope :bigger_rank, order("row_order desc")
    before_validation :assign_default_rank
  end

  def invert(target)
    a = self.row_order
    b = target.row_order
    self.row_order = target.row_order
    target.row_order = a
    if self.save 
      if target.save
        true
      else
        self.row_order = a
        self.save
        false
      end
    else
      false
    end
  end

  def increase_rank
    return false unless self.class.next_rank(self.row_order).first && 
    self.invert(self.class.next_rank(self.row_order).first)
  end

  def decrease_rank
    return false unless self.class.previous_rank(self.row_order).first && 
    self.invert(self.class.previous_rank(self.row_order).first)
  end

  private
  def assign_default_rank
    if !self.row_order
      logger.debug "/////#{self.class.bigger_rank.first.inspect}\\\\\\/"
      if self.class.bigger_rank.first
        self.row_order = self.class.bigger_rank.first.row_order + 1
      else
        self.row_order=0
      end
    end
  end
end