How To: Set up devise as a single user system

Rationale: Some projects might come across the need for an authentication solution to which devise is supremely suited for -- but without the need (or want) to have public viewers trying to register.

The example of a private weblog comes to mind, so we will use it as an example. This, along with How to: add an admin role (Especially using Option 2 of just using an :admin attribute), and lastly How To: Restrict access to specific actions gives a pretty robust authentication and authorization abilities to your app/website in mere minutes.

In order to implement our single user registration system, we are going to:

• Override the registration controller of Devise (Step 1)
• Add a method before the registration page is rendered (Step 2). This method will check if one user is already registered. The user will be redirected to the home page or # page if the application has one user. If the application has no user, the registration page will be rendered.

Step 1:

Alter the devise_for line in config/routes.rb to override the registration controller:

devise_for :users, controllers: { registrations: "registrations" }

Step 2

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
      else
        redirect_to new_user_session_path
      end
    end  
  end

end

That's it

The /users/sign_in path still gives you a login form, and /users/sign_out still logs you out.

You can add this to your application.html.erb to make some quick and easy links:

<% if user_signed_in? %>
   <%= link_to('logout', destroy_user_session_path, method: :delete) %>
<% else %>
  <%= link_to('login', new_user_session_path) %>
<% end %>