This is a quick one, and the title says most of it. Basically, you should never have code like this in your app:

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class SomeController < ApplicationController
  def some_action
    if something_about_the_url?
      do_something
      render :template => :foo
    else
      do_something_else
      render :template => :baz
    end
  end
end

The whole point of the router is to handle stuff about the url! Instead, move whatever the logic inside something_about_the_url? does upstream to the router layer. For example, say you want to display a different home page for www.mysite.com and blog.mysite.com. This can be accomplished very easily using the router:

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constraints subdomain: 'blog' do
  root to: 'blog#home'
end

root to: 'static#home'

Note that, in this specific case, you must have the subdomain route above the root route, otherwise the router will match the route to static#home before it gets to the subdomain constraint. Remember that the router checks routes in order. All set!