session_model_proxy.rb

class SessionModelProxy

  attr_reader :session_data

  def initialize(user, session_data, *args)
    if session_data.is_a?(Hash)
      @session_data = session_data
    else
      load_from_db(klass.find(id), args)
    end
  end
  
  private
  
  def load_from_db(obj, *attrs)
    @klass = obj.class
    @id = obj.id
    @attrs = {}
    (attrs << :id).uniq.each do |attribute|
      @attrs[attribute.to_sym] = obj.send(attribute.to_sym)
    end
    @session_data = @attrs
  end
  
  def method_missing(method, *args, &block)
    if @attrs[method.to_sym]
      @attrs[method.to_sym]
    else
      @obj ||= @klass.find(@attrs[:id])
      @obj.send(method, *args, &block)
      load_from_db(@obj, @attrs.keys)
    end
  end
end

module AuthSystem
  def current_user
    @current_user ||= SessionModelProxy.new(User, session[:user])
  end
  
  def login_from_session
    self.current_user = User.find_by_id(session[:user]) if session[:user]
  end
  
  def current_user=(new_user)
    
    session[:user] = (new_user.nil? || new_user.is_a?(Symbol)) ? nil : new_user.id
    @current_user = new_user
  end
end