How to properly use ActiveResource::CustomMethods when constructing URL?

Question

Currently I am using Active Resource to consume a REST API. It works fine for simple things but for some reason will not respect custom methods.

E.G. API::Model.find(m.id) #This works as expected

API::Model.find(m.id).put(:foo) #This is equivalent to doing the above

The Rails API http://api.rubyonrails.org/classes/ActiveResource/CustomMethods.html seems to imply that the second command would create a PUT to /models/:id/foo.xml but instead creates a GET to /models/:id.xml

Any ideas?