Hi --
Let's say you're building a movie rental website. You have customers, movies, and rentals to manage. REST style urls might look like this:
/customers/4 /movies/15 /rentals
You'd probably also want administrators to be able to view all of the rentals done by a particular customer, and other similar details:
/customers/4/rentals /movies/15/rentals /rentals/529/customers
Except you can't, at least not as far as I understand route generation. Looking at routes.rb for the example:
map.resources :movies do |movies| movies.resources :rentals end
map.resources :rentals do | rentals| rentals.resources :customers end
map.resources :customers do |customers| customers.resources :rentals end
Do you really want both rentals/customers and customers/rentals?
These map blocks introduce ambiguity about what rental_url(1,5) should generate - /movies/1/rentals/5 or /customers/1/movies/5. The error message pointing this out sucks (rentals_url failed to generate from {:controller=>"rentals", :action=>"index"}, expected: {:controller=>"rentals", :action=>"index"}, diff: {}), but I digress.
Yes, that error message is a source of much frustration as well as
amusement 
Instead of throwing an error in this kind of situation (which I imagine would be relatively common), why not take a named route like customer_rentals_url(1,5) or new_movie_rental_url(1) when there is ambiguity?
There may be a very good reason why not, and I'm new here, so bear with me.
I'm a little uncertain what you want the end result to be (a couple of things in the examples aren't clear to me), but it sounds like you could make use of name prefixes -- for example:
map.resources :rentals do |rentals| rentals.resources :movies, :name_prefix => "movies_" rentals.resources :customers, :name_prefix => "customers_" end
which (if I've got [untested] syntax right) should give you rentals_movies_url, rentals_customers_url, and so forth. To do it the other way around, you would do:
map.resources :movies do |movies| movies.resources :rentals, :name_prefix => "movies_" end
and the same for :customers, to get movies_rentals and customers_rentals.
David