tabletop-companion/app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  before_action :authenticate

  private

    def authenticate
      if authenticated_user = User.find_by(id: session[:user_id])
        Current.user = authenticated_user
      else
        flash[:alert] = t("not_authenticated")
        redirect_to login_path
      end
    end
end