Redirect root to /admin

I have a really strange issue, in one moment I get this working but now is always failing.

I want the /admin suffix to be the root page of my app.

I tried:

root to: redirect('/admin')
  devise_for :admin_users, ActiveAdmin::Devise.config
  ActiveAdmin.routes(self)

This:

  devise_for :admin_users, ActiveAdmin::Devise.config
  ActiveAdmin.routes(self)
  get '/', to: redirect('/admin')

And this (without luck):

  devise_for :admin_users, ActiveAdmin::Devise.config
  ActiveAdmin.routes(self)
  root to: 'admin/dashboard#index'

I think the last option was the snippet working yesterday but I'm not sure. What you suggest to do that? Any idea?