New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

helpers.is-async-belongsto-defined.js

import Ember from 'ember';
import DS from 'ember-data';

// Inputs
// params[0] - The object to check the relationship of
// params[1] - The relationship name to check
// Returns a promise object with parameter 'isAvailable' if the end of the relationship is defined

export function isAsyncBelongstoDefined(params) {
  if (params === undefined || params.length < 2 || params[0] === undefined || params[1] === undefined) {
    return false;
  }
  console.log('in');
  return DS.PromiseObject.create({
    promise: params[0].get(params[1]).then(obj => {
      return { isAvailable: obj !== undefined };
    })
  });
}

export default Ember.Helper.helper(isAsyncBelongstoDefined);

models.child.js

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

export default Model.extend({
  parent: belongsTo('parent', {async: true})
});

models.my-parent.js

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

export default Model.extend({
  name: attr('string'),
  child: belongsTo('child', {async: true})
  
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
   return this.store.createRecord('myParent', {name: 'abc'}); 
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{model.name}}
{{outlet}}
<br>
<br>
{{#if (is-async-belongsto-defined model 'child')}}
Im here

twiddle.json

{
  "version": "0.10.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.8.0",
    "ember-data": "2.8.0",
    "ember-template-compiler": "2.8.0",
    "ember-testing": "2.8.0"
  },
  "addons": {}
}