creddit/app/controllers/application_controller.rb

# controllers/application_controller.rb
class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception

  helper_method :current_user
  helper_method :current_session
  helper_method :logged_in?

  include Pundit

  rescue_from Pundit::NotAuthorizedError, with: :user_not_authorized

  private

  def current_user
    @current_user ||= User.find(current_session[:user_id]) if current_session
    @current_user ||= GuestUser.new
  end

  def current_session
    @current_session ||= UserSession.authenticate(cookies[:user_session])
  end

  def logged_in?
    current_user.registered?
  end

  def user_not_authorized
    flash[:alert] = 'You are not authorized to perform this action.'
    redirect_to(request.referrer || root_path)
  end
end