Pushing new meta-attr

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { inject as service } from '@ember/service';
export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	@service('store') store;

	constructor() {
  	super(...arguments);
    this.setup();
  }	
	@action setup() {
    this.store.pushPayload({
      included: [ 
      		{id: '1', type: 'meta-attribute', attributes: { value: 'a', name: 'firstname' } },      	          	{id: '2', type: 'meta-attribute', attributes: { value: 'b', name: 'lastname' } }

      ],
    	data: { 
        id: '1', 
        type: 'meta-model', 
        relationships: { 
          'meta-attributes': {
      			data: [
          		{id: '1', type: 'meta-attribute'},
          		{id: '2', type: 'meta-attribute'},
        		]
      		} 
      	}
    	}
    });
  	this.set('items', this.store.peekAll('meta-model'));
  }

@action pushMetaAdditive() {
	this.store.createRecord('meta-additive', { parent: this.store.peekAll('meta-attribute').firstObject });
}
 @action pushMetaAttribute() {
   const model = this.store.peekRecord('meta-model', '1');
   
 	this.store.createRecord('meta-attribute', {
    	id: Date.now().toString(),
      value: 'c',
    	name: 'middlename',
      parent: model,
  });
 }
}

models.meta-additive\.js

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

export default class extends Model {
    @belongsTo('meta-attribute', { async: false, polymorphic: true, inverse: 'additives' })
  parent;
}

models.meta-attribute\.js

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

export default class extends Model.extend({
	  countAdditives: alias('additives.length'),
}) {
  @attr('string') value;
  @attr('string') name;
  @belongsTo('meta-model', { async: false, polymorphic: true, inverse: 'metaAttributes' })
  parent;
  @hasMany('meta-additive',  { async: false, polymorphic: true, inverse: 'parent' })
  additives;
}

models.meta-model\.js

import Model from 'ember-data/model';
import { belongsTo, hasMany } from 'ember-data/relationships';
import EmberObject, { computed, defineProperty, get } from '@ember/object';
import { alias } from '@ember/object/computed';

export default class extends Model.extend({
	allAdditives: computed('[email protected]', function(){
  	return this.metaAttributes.toArray().reduce((acc, el)=>{
    	return acc.concat(el.additives.toArray());
    }, []);
  })
}) {
  @hasMany('meta-attribute', { async: false, inverse: 'parent', polymorphic: true })
  metaAttributes;
}

templates.application\.hbs

<br>
<button {{on "click" this.pushMetaAdditive}}>Push MetaAdditive</button>
<br>
<br>
<button {{on "click" this.pushMetaAttribute}}><small>Push MetaAttribute</small></button>
<br>
<br><br>
All additives count: {{this.items.firstObject.allAdditives.length}}
<br>

<br>

Issue:
<i>Computed aliases, related to relationships hasMany length is not invalidated if models count changed.</i>
<br>
Steps to reproduce:<br>

<ul>
<li>specify "ember": "3.12.0" (by default for this twiddle)</li>
<li>click on "Push MetaAdditive" button and check "All additives count" updated (for ember-source 3.12)</li>
<li>specify "ember": "3.13.0" </li>
<li>click on "Push MetaAdditive" button and check "All additives count" is NOT updated</li>
<li>click on "Push MetaAttribute" and check, "All additives count" is update</li>
</ul>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.12.0",
    "ember-template-compiler": "3.13.0",
    "ember-testing": "3.13.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-data": "3.12.5"
  }
}