positioned.rb

module Positioned
  extend ActiveSupport::Concern

  included do
    before_save :set_position, unless: :position
  end

  module ClassMethods
    def positioned
      order(:position, :id)
    end
    
    def update_positions ids
      transaction do
        ids.each_with_index do |id, i|
          position = i + 1
          where(id: id).update_all(position: position)
        end
      end
    end
  end

private
  def set_position
    self.position = Time.now.to_i # i.e., position new records at the bottom of the list
  end
end