Merb::Router.prepare do
  resources :users do
    resources :comments
  end

  resources :articles do
    resources :comments
  end
end

# While in /users/1/comments/1
url(:parents) # => /users
url(:parent, @parent) # => /users/1

# while in /articles/1/comments/1
url(:parents) # => /articles
url(:parent, @parent) # => /articles/1