Rails Routing Errors: The "diff:{}" Problem

Rails error messages are generally pretty good, but like anything, every once in a while you get one that just makes you go nuts. In creating a new section of our application, I decided to go the RESTy route and use the shiny new magic url thingies. To make life even more interesting, I used nested resources. In particular, I set up one route like this:

map.resources :calls, 
              :path_prefix => 'call_center', 
              :controller => 'call_center/calls',
              :member => { :close => :get } do |call|
  call.resources :call_notes,
                 :controller => 'call_center/call_notes',
                 :new => {:add_note => :post}
end

Unfortunately, I immediately got this error when I tried to use my new route:

  add_note_new_call_note_url failed to generate from 
   {:action=>"add_note", 
    :controller=>"call_center/call_notes"},  
  expected: 
   {:action=>"add_note", 
    :controller=>"call_center/call_notes"}, 
  --> diff: {} <-- 

Notice the last line: diff: {}. So the difference between what the routing system got and what it wanted was….nothing. The answer, which I found after much gnashing of teeth, turned out to be really simple. I needed to add a parameter to the add_note_new_call_note_url. The call that generated the error was

<% remote_form_for :call_note, 
                   :url => add_note_new_call_note_url, ...
                   

What it needed to be was

    :url => add_note_new_call_note_url(@call), ...
    

The @call provides the url generator with the correct context. I had seen (and used) this parameterized url helper in other places, but had not added it here. In fact, I'm still a little fuzzy on when exactly its needed, since some examples (even in AWDWR) don't use it and some do. At any rate, I now have a first place to look when I get this odd error message again. Lesson learned (I hope…)