post.comments.@each.id observers not called on Comment#save

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  init() {
    $.mockjax({
      type: 'get',
      url: '/posts/1',
      responseText: { 
        'data': {
          'id': '1', 
          'type': 'post',
          'attributes': {
            'name': "First post"
          },
          'relationships': {
            'comments': {
              'data': [],
              'links': {
                'related': '/posts/1/comments',
                'self': '/posts/1/relationships/comments'
              }
            }
          }
        }
      }
    });

    $.mockjax({
      type: 'post',
      url: '/comments',
      responseText: { 
        'data': {
          'id': '1', 
          'type': 'comment',
          'relationships': {
            'game': {
              'links': {
                'related': '/comments/1/game',
                'self': '/comments/1/relationships/game'
              }
            }
          }
        }
      }
    });
  },

  post: Ember.computed(function() {
    return this.store.findRecord('post', 1);
  }),

  postCommentIdsChanged: Ember.observer('post.comments.@each.id', function() {
    console.log('post.comments.@each.id observer called');
  }),

  actions: {
    buildComment() {
      this.get('post').then((post) => {
        let comment = this.store.createRecord('comment', { post: post });
        this.set('newComment', comment);
      });
    },
    saveComment() {
      let comment = this.get('newComment');
      console.log('comment will save, post.comments.@each.id should change soon');
      console.log('[' + this.get('post.comments').mapBy('id') + ']');
      comment.save().then(() => {
        console.log('comment saved, post.comments.@each.id should have changed');
        console.log('[' + this.get('post.comments').mapBy('id') + ']');
      });
    }
  }
});

application.template.hbs

{{outlet}}
<h2>{{post.name}}</h2>
<button {{action "buildComment"}}>#buildComment</button>
<button {{action "saveComment"}}>#saveComment</button>

models.comment.js

import DS from 'ember-data';

export default DS.Model.extend({
  post: DS.belongsTo('post')
});

models.post.js

import DS from 'ember-data';

export default DS.Model.extend({
  comments: DS.hasMany('comment'),
  name: DS.attr()
});

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js",
    "jquery-mockjax": "https://cdnjs.cloudflare.com/ajax/libs/jquery-mockjax/1.6.2/jquery.mockjax.js"
  }
}