belongsTo test

controllers.application.js

import Ember from 'ember';

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

models.child-a.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'),
  name: attr('string')
});

models.child-b.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'),
  name: attr('string')
});

models.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'),
  childA: belongsTo('child-a'),
  childB: belongsTo('child-b')
});

router.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('test', { path: '/' });
  this.route('test');
});

export default Router;

routes.test.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    let parent = this.store.peekRecord('parent', 1);
    if (!parent) {
      this._setup();
      parent = this.store.peekRecord('parent', 1);
      console.log(parent.childA.get('name'));
      return parent;
    } else {
      return parent;
    }
  },
  
  _setup() {
  	this.store.push({
  		data: [
        {
          id: 1,
          type: 'child-a',
          attributes: {
            name: 'Child A'
          }
        },
        {
          id: 1,
          type: 'child-b',
          attributes: {
            name: 'Child B'
          }
        },
      ]
    });
    this.store.push({
      data: [
        {
					id: 1,
          type: 'parent',
          attributes: {
            name: 'Parent'
          },
          relationships: {
            childA: {
              data: {
                id: 1,
                type: 'child-a'
              }
            },
            childB: {
              data: {
                id: 1,
                type: 'child-b'
              }
            }
          }
        }
      ]
    });
	}
});

templates.application.hbs

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

<p>outlet</p>
<br>
{{outlet}}
<br>
<p>/outlet</p>

templates.test.hbs

<p>Hello</p>
<div>
{{model.childA.name}}
</div>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.2"
  }
}