Ember-data and MongoDB, how to handle _id

I'm using ember-data with rails and MongoDB and am having problem with the way IDs are stored in MongoDB - in a _id field.

Ember-data will use id as the default field for ID so I tried to override it like this:

App.User = DS.Model.extend
    primaryKey: "_id"
    name: DS.attr "string"
    image: DS.attr "string"

This seems to work most of the time but in some instances I get exceptions from ember saying:

Uncaught Error: assertion failed: Your server returned a hash with the key _id but you have no mappings

I suspect this might be a bug in ember-data because it's still heavily under development, but I was trying to find a way to get to map _id to id on the server side in rails? I'm using mongoid to do the mongo mapping.