New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  actions: {
    createComment() {
      let post = this.get('model');
      let comment = this.get('store').createRecord('comment', {post});
      post.set('comment', comment);
    },
    
    deleteComment() {
      let post = this.get('model');
			post.get('comment').then(comment => {
        comment.deleteRecord();
      });
    }
  }
    
  
});

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({
  post: belongsTo('post')
});

models.post.js

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

export default Model.extend({
  comment: belongsTo('comment'),
  
  // false positive
  hasComment1: Ember.computed('comment', function() {
    return Ember.isPresent(this.get('comment'));
  }),
  
  // Doesn't work for non persisted comments
  hasComment2: Ember.computed('comment.id', function() {
    return Ember.isPresent(this.get('comment.id'));
  }),
  
  // uses 'private' content
  hasComment3: Ember.computed('comment', function() {
    return Ember.isPresent(this.get('comment.content'));
  }),
  
  hasComment4: Ember.computed('comment', function() {
    return Ember.isPresent(this.belongsTo('comment'));
  })
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.get('store').createRecord('post');
  }
});

templates.application.hbs

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

<button {{action 'createComment'}}>Create comment</button>
<button {{action 'deleteComment'}}>Delete comment</button>

<p>model.hasComment1: {{model.hasComment1}}</p>
<p>model.hasComment2: {{model.hasComment2}}</p>
<p>model.hasComment3: {{model.hasComment3}}</p>
<p>model.hasComment4: {{model.hasComment4}}</p>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}