module TimestampedColumn
  module ActiveRecord
    extend ActiveSupport::Concern

    included do
      before_save    :update_timestamped_column_times
      cattr_accessor :timestamped_columns
    end

    module ClassMethods
      def timestamped_column(column, opts={})
        timestamp_column = opts[:column] || "#{column}_updated_at"

        self.timestamped_columns ||= {}
        self.timestamped_columns[column] = timestamp_column
      end
    end

    module InstanceMethods
    private
      def update_timestamped_column_times
        if self.timestamped_columns && changes.any?
          self.timestamped_columns.each do |column, timestamp_column|
            if changes[column.to_s]
              send "#{timestamp_column}=", Time.zone.now
            end
          end
        end
      end
    end
  end
end

class ActiveRecord::Base
  include TimestampedColumn::ActiveRecord
end