How do I use Devise and ActiveAdmin for the same User model?

Question

I have ActiveAdmin and Devise working with Users. I would like to use Devise to log in regular non-admin users with the same User model. How can I do this? (I want to have an admin flag in the User model for only admins.) I tried adding the 2nd line to routes.rb

devise_for :users, ActiveAdmin::Devise.config
devise_for :users

But it gave an error when I tried to list the routes

>rake routes
DL is deprecated, please use Fiddle
rake aborted!
ArgumentError: Invalid route name, already in use: 'new_user_session'
You may have defined two routes with the same name using the `:as` option, or you may be overriding a route already defined by a resource with the same naming. For the latter, you can restrict the routes created with `resources` as explained here:
http://guides.rubyonrails.org/routing.html#restricting-the-routes-created

I've created an authorization adapter which just checks user.admin == true and that is working OK for ActiveAdmin. https://github.com/activeadmin/activeadmin/blob/master/docs/13-authorization-adapter.md

Answer 1

I found this http://dan.doezema.com/2012/02/how-to-implement-a-single-user-model-with-rails-activeadmin-and-devise/

But I ended up doing this

Devise 3.4.1
ActiveAdmin 1.0.0.pre1
Rails 4.2.1

routes.rb

  devise_for :admin_users, {class_name: 'User'}.merge(ActiveAdmin::Devise.config)
  ActiveAdmin.routes(self)

  devise_for :users
  resources :users

application_controller.rb

  def access_denied(exception)
    redirect_to root_path, alert: exception.message
  end

config/initializers/active_admin.rb

config.authorization_adapter = ActiveAdminAdapter
config.on_unauthorized_access = :access_denied

(And changing all methods from _user to admin_user.)

app/models/active_admin_adapter.rb

class ActiveAdminAdapter < ActiveAdmin::AuthorizationAdapter
  def authorized?(action, subject = nil)
    user.admin == true
  end
end

And

rails generate migration add_admin_to_users admin:boolean