application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  protected

  def current_user
    @current_user ||= User.find_by_id(session[:user_id]) if signed_in?
  end

  def signed_in?
    !! session[:user_id]
  end

  helper_method :current_user, :signed_in?

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

  def logout!
    @current_user = nil
    session.delete :user_id
  end
end