New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  showHasComments: false,
  showHasCommentsTriggersFetch: false,
  
  actions: {
    toggle(propertyName) {
      this.toggleProperty(propertyName);
    }
  }
});

models.comment.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
});

models.my-model.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  title: DS.attr('string'),
  body: DS.attr('string'),
  comments: DS.hasMany('comment', { async: true} ),

  hasComments: Ember.computed('comments', function() {
    return this.hasMany('comments').ids() > 0;
  }),
  
  hasCommentsTriggersFetch: Ember.computed.gt('comments.length', 0)
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model: function() {
    this.store.push({
      data: [{
        id: 1,
        type: 'my-model',
        attributes: {
          "title": "some cats are cool",
          "body": "",
        },
        relationships: {
          "comments": {
            "data": [{"id": "5", "type": "comment"}]
          }
        }
      }]
    });
    return this.store.peekRecord('my-model', 1);
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{model}}
<br><br>

<div>Won't trigger an async fetch for comments</div>
<button {{action "toggle" "showHasComments"}}>Show hasComments</button>
{{#if showHasComments}}
  {{model.hasComments}}
{{/if}}

<br><br>

<div>Will trigger an async fetch for comments (check out console)</div>
<button {{action "toggle" "showHasCommentsTriggersFetch"}}>Show hasCommentsTriggersFetch</button>
{{#if showHasCommentsTriggersFetch}}
  {{model.hasCommentsTriggersFetch}}
{{/if}}

<br>

twiddle.json

{
  "version": "0.10.7",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.0",
    "ember-data": "2.5.5",
    "ember-template-compiler": "2.10.0",
    "ember-testing": "2.10.0"
  },
  "addons": {}
}