How to namespace controllers for subdomains?

I'm trying to separate my resources such that any http://subdomain.root.com uses a different set of controllers and views.

I read about using constraints for subdomain like so:

namespace :admin do
  constraints subdomain: 'admin' do
    resources :photos
  end
end

How would I configure resources for any/all subdomains? I don't care what the subdomain is, just that a subdomain is being used.

I thought  about putting a method in the routes file to check for presence of request.subdomain but this felt like a hack.

You'll need to setup a route constraint to filter routes for subdomains, then you can add a scope module: :subdomain do block around the routes that are for the subdomains only. 

constraints subdomain: :admin do
  scope module: :subdomain do
    resources :posts
  end
end

And that should generate routes that only respond on the subdomain, and are automatically mapped to controllers with the "subdomains" namespace.