It doesn’t look like Ember Data supports nested REST resources at the moment. There’s been some discussions around it and a few proposed solutions, but nothing that’s actually shipped as of now (that I know of).
This issue on the Ember Data repo discussed it, and the last reply from Yehuda states (6 months ago) that “I’ll keep it in mind going forward.”
This discussion on these forums also goes through some reasons why nested resources might not be the best solution anyway, as well as possible solutions.
You could probably find some temporary solution by overriding buildUrl in your adapters.