New Twiddle

controllers.application.js

import Ember from 'ember';
import Component from '@ember/component';

export default Ember.Controller.extend({});

models.author.js

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

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

models.book.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: attr('string'),
  authors: hasMany('author',{async:true}),
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
		const book = this.store.createRecord('book');
    book.authors.createRecord();
    return book;
  }
});

templates.application.hbs

{{this.model.authors.length}}

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": "3.7.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.7.0"
  }
}