config.ru

require "intercession"
require "myapp"
require "myapp/session_extras"

use Rack::Session::Cookie, :secret => "I'll punchasize your face for FREE!"
use Intercession, MyApp::SessionExtras

run MyApp

intercession.rb

class Intercession
  def initialize app, behavior
    @app      = app
    @behavior = behavior
    
    methods   = @behavior.instance_methods.map(&:to_s)
    @before   = !!methods.include? "before"
    @after    = !!methods.include? "after"
  end

  def call env
    if session = env["rack.session"]
      session.extend @behavior
    end

    session.before if @before
    result = @app.call env
    session.after if @after

    result
  end
end

myapp.rb

class MyApp < Sinatra::Base
   get "/" do
     halt 403 if session.anonymous?
     "Yay, I know you!"
   end
end

session_extras.rb

class MyApp
  module SessionExtras
    def user
      @user ||= self[:user_id] && User.find_by_id(self[:user_id])
    end

    def anonymous?
      !user
    end
  end
end