Just to clear things up a bit (as I remember not knowing about this side of "RESTfulness" a while ago myself): an important part of REST is relying on hypertext

Just to clear things up a bit (as I remember not knowing about this side of "RESTfulness" a while ago myself): an important part of REST is relying on hypertext, that is pointing to other resources by using URLs. Just returning an ID of a referenced object/resource as a plain number isn't very RESTful, but returning an URL of that object/resource is.