Difference in Promised Properties

adapters.application.js

import DS from 'ember-data';

export default DS.FixtureAdapter.extend({});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  comments: Ember.computed('model.comments',function(){
    return this.get('model.comments');
  }),
  resolvedComments: Ember.computed('model.comments',{
    get(key) {
      this.get('model.comments').then(function(_this) {
        return function(comments) {
          _this.set('resolvedComments',comments);
          return;
        };
      }(this));
      return false;
    },
    set(key,value) {
      return value;
    }
  })
});

models.comment.js

import DS from "ember-data";

var Comment = DS.Model.extend({
  body: DS.attr('string'),
  author: DS.attr('string'),
  post: DS.belongsTo('post',{async:true})
});

Comment.reopenClass({
  FIXTURES: [
      {
        id: 1,
        author: "Zach Wentz",
        post: 1,
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      },
      {
        id: 2,
        author: "Ryan Christiani",
        post: 1,
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      },
      {
        id: 3,
        author: "Ryan Christiani",
        post: 1,
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      }
  ]
});

export default Comment;

models.post.js

import DS from "ember-data";

var Post = DS.Model.extend({
  title: DS.attr('string'),
  body: DS.attr('string'),
  author: DS.attr('string'),
  comments: DS.hasMany('comment',{async:true})
});

Post.reopenClass({
  FIXTURES: [
      {
        id: 1,
        title: "Using Ember CLI to create a Fixture Adapter.",
        author: "Zach Wentz",
        comments: [1,2,3],
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      },
      {
        id: 2,
        title: "Ember is lots of fun",
        author: "Ryan Christiani",
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      },
      {
        id: 3,
        title: "Ember, Node, Express and You!",
        author: "Ryan Christiani",
        body: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium aspernatur quam qui commodi beatae placeat ducimus aliquam veritatis ullam sed! Sit assumenda aspernatur sunt harum accusamus, repellat labore! Repellendus, corporis!"
      }
  ]
});

export default Post;

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  store: Ember.inject.service(),
  model(params) {
    return this.get('store').find('post',1);
  }
});

templates.application.hbs

<h1>Promises in Computed Properties</h1>
<br>
<br>
Comments: {{comments}}
<br>
<br>
Resolved comments: {{resolvedComments}}

twiddle.json

{
  "version": "0.9.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "1.13.15",
    "ember-template-compiler": "2.6.0"
  }
}