I've read the B-Simple REST tutorial (http://www.b-simple.de/documents) which explains this process for one-to-many associations. But things are a little different for me, because I only want one avatar per user, I do not need an index action and the urls do look different. For example, I want /user/:user_id/avatar to show the user avatar, /user/:user_id/avatar;edit to change the avatar etc.
map.resources :people do |people| people.resource :avatar end
avatar_url(:person_id => person) # => /people/5/avatar edit_avatar_url(:person_id => person) # => /people/5/avatar;edit