Touch plugin for MongoMapper

gistfile1.rb

# encoding: UTF-8
module MongoMapper
  module Plugins
    module Touch
      extend ActiveSupport::Concern

      module InstanceMethods
        def touch(key = :updated_at) 
          raise "InvalidKey" unless self.key_names.include?(key.to_s) 
          self.set(key => Time.now.utc)

          true
        end 
      end
    end
  end
end

But I prefer to store Dates/Times in UTC