Rails Ruby on Rails

Suppose we have routes defined as

  constraints :subdomain => 'apples' do
    resources :apples
  end
  constraints :subdomain => 'oranges' do
    resources :oranges
  end

And in oranges#show (accessed from http://oranges.example.com/oranges/1 ) we need a link to an apple. Writing

  <%= link_to @apple.name, @apple %>

won't fail until you actually click the link and get the 404 error, because the link would have href="http://oranges.example.com/apples/1". So the suggestion is to consider current subdomain in url generation and if it doesn't satisfy constraints in routes raise error early.

By quick reading of url_for source code I didn't get the idea how to implement it correctly. Any suggestions on this?