application_controller.js.coffee

App.ApplicationController = Ember.ObjectController.extend
  order: null
  product: null
  init: ->
    order_uuid = $('meta[name="order_uuid"]').attr('content')
    @store.find("order", order_uuid).then((order) =>
      console.log "Yay, got", order
      order.get("product").then((product) =>
        console.log "sad, did not get", product
        # I get this error in the console instead:
          # TypeError: Cannot read property 'then' of null
          # at http://192.168.33.12:3000/assets/controllers/application_controller.js?body=1:11:38
          # at invokeCallback (http://192.168.33.12:3000/assets/ember.js?body=1:9754:19)
          # at publish (http://192.168.33.12:3000/assets/ember.js?body=1:9424:9)
          # at Promise.publishFulfillment (http://192.168.33.12:3000/assets/ember.js?body=1:9844:7)
          # at Object.DeferredActionQueues.flush (http://192.168.33.12:3000/assets/ember.js?body=1:5894:24)
          # at Object.Backburner.end (http://192.168.33.12:3000/assets/ember.js?body=1:5985:27)
          # at Object.Backburner.run (http://192.168.33.12:3000/assets/ember.js?body=1:6024:18)
          # at Object.Ember.run (http://192.168.33.12:3000/assets/ember.js?body=1:6427:26)
          # at hash.success (http://192.168.33.12:3000/assets/ember-data.js?body=1:1524:19)
          # at fire (http://192.168.33.12:3000/assets/jquery.js?body=1:3100:30) 

      )
    )

ember.log

DEBUG: Ember      : 1.4.0 ember.js?body=1:3462
DEBUG: Ember Data : 1.0.0-beta.7+canary.f482da04 ember.js?body=1:3462
DEBUG: Handlebars : 1.3.0 ember.js?body=1:3462
DEBUG: jQuery     : 1.11.0 

order.js.coffee

App.Order = DS.Model.extend
  uuid: DS.attr("string")
  address: DS.attr("string")
  product: DS.belongsTo("product", async: true)

order.json

{"order":{"id":2,"uuid":"89069589-5750-41b7-a91e-18da7c03e0c3","product_id":2}}

order_serializer.js.coffee

App.OrderSerializer = DS.RESTSerializer.extend
  primaryKey: "uuid"

product.js.coffee

App.Product = DS.Model.extend
  name: DS.attr("string")
  orders: DS.hasMany("order", async: true)

product_2.json

{"product":{"id":2,"name":"Secret Guide to Secrets","description":"This is the only book you'll ever need.","order_ids":[1,2]}}