Device gem with one user allowed

registrations_controller.rb

# app/controllers/registrations_controller.rb
class RegistrationsController < Devise::RegistrationsController

  before_action :one_user_registered?, only: [:new, :create]
  
  protected

  def one_user_registered?
    if User.count == 1
      if user_signed_in?
        redirect_to root_path, alert: 'Регистрация новых пользователей отключена'
      else
        redirect_to new_user_session_path, alert: 'Регистрация новых пользователей отключена'
      end
    end  
  end

end