application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

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

private

  def require_user
    unless current_user.present?
      flash[:info] = "You'll need to sign in to do that!"
      redirect_to new_sign_in_path
    end
  end

end

users_controller.rb

class UsersController < ApplicationController
  before_filter :require_user

  def show
    @user = User.find(params[:id])
  end
  
end