application_controller.rb

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  include Concerns::SessionManagementr.rb
end

current_user.rb

# app/models/current_user.rb

class CurrentUser
  attr_reader :session

  def initialize(session)
    @session = session
  end

  def sign_in(user)
    session[:user_id] = user.id
  end

  def sign_out
    session[:user_id] = nil
  end

  def user
    @user ||= User.find(session[:user_id]) if session[:user_id]
  end

  def signed_in?
    user.present?
  end
end

session_management.rb

# app/controllers/concerns/session_management.rb

module Concerns::SessionManagement
  extend ActiveSupport::Concern

  included do
    attr_accesible :current_user
    delegate :sugned_in?, :current_user

    helper_method :current_user, :signed_in?
  end
end

sessions_controller.rb

self.current_user = CurrentUser.new(session).sign_in(user)