New Twiddle

controllers.application.js

import Ember from 'ember';

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

controllers.aroute.js

import Ember from 'ember';

export default Ember.Controller.extend({
  actions: {
    returnToIndex() {
      this.transitionToRoute('index');
    }
  }
});

controllers.index.js

import Ember from 'ember';

export default Ember.Controller.extend({
  myOutter: null,
  activeInners: Ember.computed.filterBy('myOutter.modelInners', 'isActive', true),
  init: function() {
    this.set('myOutter', this.store.createRecord('model-outter', {}));
  },
  actions: {
    addInner() {
      let myInner = this.store.createRecord('model-inner',  {isActive: true, modelOutter: this.get('myOutter')})
    },
    removeLastInner() {
      let lastInner = this.get('activeInners.lastObject');
      console.log('unloading '+lastInner);
      this.store.unloadRecord(lastInner);
    },
    gotoARoute() {
      this.transitionToRoute('aroute');
    }
  }
});

models.model-inner.js

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

export default Model.extend({
  modelOutter: belongsTo('model-outter', {async: true}),
  isActive: null,
  unloadRecord() {
    this.set('modelOutter', null);
    return this._super(...arguments);
  }
});

models.model-outter.js

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

export default Model.extend({
  modelInners: hasMany('model-inner', {async: true, inverse: 'modelOutter'}) 
});

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('index', {path: '/'});
  this.route('aroute');
});

export default Router;

routes.aroute.js

import Ember from 'ember';

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

routes.index.js

import Ember from 'ember';

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

templates.application.hbs

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

templates.aroute.hbs

Nothing to see here
<br>

<button {{action 'returnToIndex'}}>Return to index</button>

templates.index.hbs

<br>
My Inners: {{myOutter.modelInners.length}}
<br/>
My Active Inners: {{activeInners.length}}
<br/>
First Active Inner: {{activeInners.firstObject}}
<br>
Last Active Inner: {{activeInners.lastObject}}
<br>
<button {{action 'addInner'}}>Add Inner</button>
<br>
<button {{action 'removeLastInner'}} disabled={{if activeInners.length null 'disabled'}}>Remove Last Inner</button>
<br>
<button {{action 'gotoARoute'}}>Go To A Route</button>

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": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}