sinatra_auth_engine.rb

class AuthEngine < Sinatra::Base

  set :sessions => true

  register do
    def auth type
      condition do
        redirect "/login" unless send "is_#{type}?"
      end
    end
  end

  helpers do
    def is_user?
      !!@user
    end
  end

  before do
    @user = User.find session[:user_id]
  end

  post '/login' do
    user = User.authenticate params

    if user then
      session[:user_id] = user.id
    else
      redirect to('/login')
    end
  end

  get '/logout' do
    session[:user_id] = nil
    redirect '/'
  end

end