routes.rb

Foo::Application.routes.draw do |map|
  match '/:id', :to => 'users#profile', :as => :profile, :via => 'get'
end

shell session

$ rspec spec -cfd

UsersController
  profile
    renders the profile url

users_controller.rb

class UsersController < ApplicationController
  def profile
    render :text => profile_url("foo")
  end
end

users_controller_spec.rb

require 'spec_helper'

describe UsersController do
  describe 'profile' do
    it 'renders the profile url' do
      get :profile, :id => "foo"
      response.body.should == "http://test.host/foo"
    end
  end
end