user.rb

class User < ActiveRecord::Base
  def access_time=(time = Time.now)
    raise "#{self.class} isn't saved." unless id

    access_log = UserAccessLog.find(id)
    access_log.time = time
    access_log.save
  end

  def access_time
    return nil unless id

    access_log = UserAccessLog.find(id)
    access_log.time
  end
end

user_access_log.rb

require 'tokyotyrant'

class UserAccessLog
  @@connection = nil
  @@tt_server = {
    :host => "localhost",
    :port => "1978",
  }

  attr_accessor :time

  def initialize(user_id, time)
    @user_id = user_id
    @time = time
  end

  def save
    @@connection.put(Digest::SHA1.hexdigest(@user_id.to_s), time.to_s)
  end

  def save!
    raise "Can't save." unless self.save
  end

  class << self
    # setup connection
    def connect
      unless @@connection
        @@connection = TokyoTyrant::RDB.new
        unless @@connection.open(@@tt_server[:host], @@tt_server[:port])
          raise "Can't connect ttserver. (#{@@connection.ecode})"
        end
      end
    end

    # find object from TokyoTyrant
    def find(user_id)
      raise "id does not specified." unless user_id

      # connect to ttserver
      connect

      access_time = @@connection.get(Digest::SHA1.hexdigest(user_id.to_s))
      access_time = nil if access_time.blank?
      self.new(user_id, (access_time ? Time.parse(access_time) : nil))
    end
  end
end