HasMany Explosion

adapters.application.js

import DS from 'ember-data';
 
export default DS.RESTAdapter.extend({});

controllers.application.js

import Ember from 'ember';

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

models.asset.js

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

export default Model.extend({
  name: attr('string'),
  positions: hasMany('position', { async: false }),
  positionsSorted: sort('positions', 'positionsSortedBy'),
  positionsSortedBy: ['utc:desc']
});

models.position.js

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

export default Model.extend({
  utc: attr('date'),
  asset: belongsTo('asset', {async: true, inverse: 'positions'}),
});

routes.application.js

import Ember from 'ember';
import { hash } from 'rsvp';

export default Ember.Route.extend({
  init(){
    this._super(...arguments);
  },  
  model(){
    // Push assets where no relationships are defined
  	this.store.pushPayload('asset',{
      assets: [{ id: 1, name: 'My Great Asset', positions: [] }]
    });
    return hash({
      assets: this.store.peekAll('asset'),
      positions: this.store.peekAll('position')
    }); 
  },
  actions: {
  	loadPositions(){
      let date = Date.now();
      this.store.pushPayload('position',{
      	positions: [{ id: date, utc: date, asset: 1 }]
    	});
    },
    cleanupPositions(){
    	const positions = this.store.peekAll('position').sortBy('utc').reverse();
      let positionCount = positions.length;
      
      while(positionCount > 1){
        let record = positions.popObject();
      	this.store.unloadRecord(record);
        positionCount--;
      }
    }
  }
});

serializers.application.js

import DS from 'ember-data';

export default DS.RESTSerializer.extend({});

templates.application.hbs

<h1>Ember Data HasMany Explosion</h1>

<h3>Loaded Assets</h3>
{{#each model.assets as |asset|}}
	{{asset.name}}
  <ul>
  	<!-- Change this line to asset.positions to see the undefined elements being populated -->
  	{{#each asset.positionsSorted as |pos|}}
    	<li>{{pos.utc}}</li>
    {{/each}}
  </ul>
{{/each}}

<h3>Loaded Positions</h3>
<ul>
{{#each model.positions as |pos|}}
	<li>{{pos.utc}}</li>
{{/each}}
</ul>
<hr />
<p>
To reproduce the issue click 'Load Positions' a couple times and then click 'cleanup positions'. The console will then puke an error about not being able to update a watcher on null/undefined item. After this point if you try and 'Load Positions' it will be unable to do so and will continue to give the same undefined error.
</p>
<button type="button" {{action 'loadPositions'}}>Load Positions</button>
<button type="button" {{action 'cleanupPositions'}}>Cleanup Positions</button>

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": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2",
    "jquery-mockjax": "https://cdnjs.cloudflare.com/ajax/libs/jquery-mockjax/1.6.2/jquery.mockjax.js"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}