application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception

  concerning :Authentication do
    included do
      before_action :authenticate_user!
    end

    private
    def authenticate_user!
      unless current_user
        redirect_to root_path
      end
    end

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