Using Subdocument as ember model

I am trying to set up a one-to-many relationship between a mongoDB Document and its Subdocuments. So far I git my models:

user/model.js:

export default DS.Model.extend({
  name: DS.attr('string'),
  age: DS.attr('number'),
  calendarEntries: DS.hasMany('calendarEntries', {embedded: true}) 
                                          //also tried embedded: 'always'
});

calendar-entry/model.js

export default DS.Model.Extend({
  title: DS.attr('string'),
  date: DR.attr('string'),
  user: belongsTo('user')  //Also tried without this
});

What I get from the api is:

"user": {
  "_id": "56e3510e8a87920cc961faf5",
  "name": "John Doe",
  "age": 25,
  "calendarEntries": [
    {
      "_id": "56e3511f8a87920cc961faf6",
      "title": "Meet friends",
      "date": "2016-03-13T00:54:13.442Z"
    }
  ]
}

I also use a serializer to set the primaryKey to '_id'. The api has the End-Points /users, /users/:userid, /users/:userid/calendarEntries and /users/:userid/calendarEntries/:entryid. In other posts i read this was working and I dont get my mistake. The error message doesnt really tell me anything: "Passing classes to store methods has been removed. Please pass a dasherized string instead of undefined". Since the other posts I read are about two years old, i dont know what changed in ember-data.