model bug

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  store: Ember.inject.service(),
  
  init(){
    this._super(...arguments);
    let entity = 
    
    this.store.pushPayload({
  data: {
    // primary data for single record of type `Person`
    id: '1',
    type: 'entity',
    attributes: {
      b: 42      
    }
  }
})
    
    console.log(entity.get('rel.isDirty'));
  }
});

mixins.model-dirty-mixin.js

import { isPresent } from '@ember/utils';
import { get } from '@ember/object';
import Mixin from '@ember/object/mixin';

export default Mixin.create({

	isModelDirty() {


		let changed = Object.keys(this.changedAttributes());

		let isDirty = false;


		get(this.constructor, 'attributes').forEach(attr =>{
			if (attr.isAttribute === true && isPresent(attr.options) && attr.options.ignoreDirty !== true){

				if (changed.includes(attr.name)){
					isDirty = true;
				}

			}
		});

		return isDirty;
	}

});

models.entity.js

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

export default Model.extend({
  rel: belongsTo('entity2'), 
  
  ready(){
    this._super(...arguments);
    
    let store = getOwner(this).lookup('service:store');
    this.set('rel', store.createRecord('entity2'));
		this.set('rel.isDirty', false); 
  }
});

models.entity2.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';
import { getOwner } from '@ember/application';
import ModelDirtyMixin from "../mixins/model-dirty-mixin";
import { computed } from '@ember/object';

export default Model.extend(ModelDirtyMixin,{
  hello:attr('string'),
  rel2: belongsTo('entity3'),
  
  	isDirty: computed(function () {
		return this.isModelDirty();
	}).volatile(),
  
  
      ready(){
    this._super(...arguments);
    
    let store = getOwner(this).lookup('service:store');
    this.set('rel2', store.createRecord('entity3'));
		this.set('rel2.isDirty', false); 
  }
});

models.entity3.js

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

export default Model.extend({
  a: attr('string')
  
});

templates.application.hbs

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

tests.helpers.destroy-app.js

import { run } from '@ember/runloop';

export default function destroyApp(application) {
  run(application, 'destroy');
}

tests.helpers.resolver.js

import Resolver from '../../resolver';
import config from '../../config/environment';

const resolver = Resolver.create();

resolver.namespace = {
  modulePrefix: config.modulePrefix,
  podModulePrefix: config.podModulePrefix
};

export default resolver;

tests.helpers.start-app.js

import Ember from 'ember';
import Application from '../../app';
import config from '../../config/environment';

const { run } = Ember;
const assign = Ember.assign || Ember.merge;

export default function startApp(attrs) {
  let application;

  let attributes = assign({rootElement: "#test-root"}, config.APP);
  attributes = assign(attributes, attrs); // use defaults, but you can override;

  run(() => {
    application = Application.create(attributes);
    application.setupForTesting();
    application.injectTestHelpers();
  });

  return application;
}

tests.test-helper.js

import resolver from './helpers/resolver';
import {
  setResolver
} from 'ember-qunit';
import { start } from 'ember-cli-qunit';

setResolver(resolver);
start();

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "2.18.0",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.3.0"
  }
}