You are one step ahead though. How can you address a resource if you don't know beforehand the representation of its identifier? Say, you want to create a new Employee under /Employees. Do you just issue a 'GET /Employees/1234' and assume '1234' is a valid identifier when you get a 404? What if the system expects a MongoDB _id such as '47cc67093475061e3d95369d'?

A workaround, as was suggested by an other poster, is that the client interrogates the system for a valid identifier and then proceeds. This especially makes sense when, as noted, you are dealing with resources that belong to no collection.