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.
The client shouldn't know how to assemble URLs. The better way to do this is to POST to /Employees and have the server return the URL of the newly-created employee.
However, in the case where the Id is the nature key (e.g. SSN), the Id can be known from the user input. Creating an employee is just a matter of PUT his full record along with his SSN as the Id.
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.