##controller
  class User < R '/admin/user'
    def get
      if @state[:user_id]
        @user = Models::User.find(@state[:user_id])
        @presentations = @user.presentation
        render :show_user
      else
        redirect Login
      end
    end
  end

  class Login < R '/admin/login'
    def get
      render :login
    end

    def post
      user = Models::User.find_by_name(input[:user])
      if user.password == input[:password]
        @state[:user_id] = @user.id
        redirect User, user.id
      else
        render :user_form
      end
    end
  end

##view
  def login
    h1 'Login'
    _user_form(R(Login))
  end

  def _user_form(action)
    form(:method => 'post', :action => action) do
      div.form_user!.form_block do
        label 'Username', :for => 'user'; br
        input :type => 'textfield', :name => 'user'
      end

      div.form_password!.form_block do
        label 'Password', :for => 'password'; br
        input :type => 'password', :name => 'password'
      end
      input :type => 'submit'
    end
  end
##error
Presenter::Controllers::Login.POST
TypeError can't dup NilClass:

    * (eval):3:in `dup'
    * (eval):3:in `R'
    * (eval):5:in `URL'
    * (eval):12:in `redirect'
    * /Users/cdcarter/software/presenter1/presenter/presenter.rb:172:in `post'
    * (eval):24:in `send'
    * (eval):24:in `service'
    * /usr/local/lib/ruby/gems/1.8/gems/camping-1.4.157/lib/camping/session.rb:112:in `service'
    * (eval):43:in `run'