Redirect root_path in rails for already signed_in user without using Devise

In my application i have root_path root 'home#mainPage' and though a user is signed in , he can accesss http://localhost:3000/ which i dont want. So i am following http://ift.tt/18j0qcZ answer to add this feature and i am getting undefined method user_signed_in? for AuthenticatedUser:Class. I am using Rails 4.2.0

My routes:

  constraints(AuthenticatedUser) do 
    root :to => "users#show", as: :authenticated
  end

  root 'home#mainPage'

Lib/authenticated_user.rb:

class AuthenticatedUser
  def self.matches?(request)
    user_signed_in?
  end
end

application_helper.rb

module ApplicationHelper
    def user_signed_in?
        !!session[:user_id]
    end
    def current_user
        User.find(session[:user_id])
    end
end

Config/application.rb

config.autoload_paths << Rails.root.join('lib')