stackoverflow question#4311517

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
	categories: {
      'Bourbons': ['Bulleit', 'Four Roses', 'Woodford Reserve'],
      'Ryes': ['WhistlePig', 'High West']
   },
  
  actions: {
    additem() {
      let categories = this.get('categories');
      categories['category'] = ['foo'];
      // A manual re-render causes the DOM to be updated
      this.rerender();
    }
  }
});

controllers.application.js

import Ember from 'ember';

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

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{my-component}}
<br>
<br>

templates.components.my-component.hbs

{{#each-in categories as |category products|}}
   <li>{{category}}
   	<ol>
    	{{#each products as |product|}}
     		<li>{{product}}</li>
      {{/each}}
   	</ol>
   </li>
{{/each-in}}

<button onclick={{action 'additem'}}>Add item to categories object</button>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-template-compiler": "2.6.0",    
    "ember-testing": "2.6.0"
  },
  "addons": {
  }
}