The Problem

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',

  selectedIds: Ember.computed('signingPackages.@each.ids', function() {
  	var ids = [];
    this.get('signingPackages').forEach(function(signingPackage) {
			ids = ids.concat(signingPackage.get('ids'));
    });
    return ids;
  }),
  
  signingPackages: [
  	Ember.Object.create({
    	ids: [1],
      items: [
        Ember.Object.create({id: 1, amount: 1}),
        Ember.Object.create({id: 2, amount: 1}),
        Ember.Object.create({id: 3, amount: 1})
      ]
    }),
    
    Ember.Object.create({
    	ids: [2],
      items: [
        Ember.Object.create({id: 4, amount: 1}),
        Ember.Object.create({id: 5, amount: 1}),
        Ember.Object.create({id: 6, amount: 1})
      ]
    })
  ],
 
  actions: {
    update: function() {
      this.get('signingPackages.firstObject').get('ids').pushObject(9)
    },
    add: function() {
      this.get('signingPackages').pushObject(
        Ember.Object.create({
	    	ids: [3],
  	    items: [
    	    Ember.Object.create({id: 1, amount: 1}),
      	  Ember.Object.create({id: 2, amount: 1}),
        	Ember.Object.create({id: 3, amount: 1})
	      ]
	    }))
    }
  }
});

templates.application.hbs

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

<h3>{{selectedIds.length}}</h3>


<ul>
{{#each signingPackages as |signingPackage|}}
	<li>Ids: {{signingPackage.ids}}, Items: {{signingPackage.items.length}}</li>
{{/each}}
</ul>

<button {{action 'add'}}>Add</button>
<button {{action 'update'}}>Update</button>
<br>
<br>

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"
  }
}