Updating object keys for each-in

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  willRender() {
    // Set the "categories" property to a JavaScript object
    // with the category name as the key and the value a list
    // of products.
    this.set('categories', {
      'Bourbons': ['Bulleit', 'Four Roses', 'Woodford Reserve'],
      'Ryes': ['WhistlePig', 'High West']
    });
  },
  
  actions: {
    addCategoryBroken(category) {
      // This won't work!
      let categories = this.get('categories');
      categories[category] = [];
    },

    addCategoryPR(category) {
      let categories = this.get('categories');
      Ember.set(categories, category, []);
    },
    
    addCategory(category) {
    	this.set(`categories.${category}`, []);
  	}
  }
});

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>
{{my-component}}
<br>
<br>

templates.components.my-component.hbs

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

{{input value=newCategory}}
<br>
<button onclick={{action 'addCategoryBroken' newCategory}}>
  Add Category (PR "broken" example)
</button>
<br>
<button onclick={{action 'addCategoryPR' newCategory}}>
  Add Category (PR "working" example)
</button>
<br>
<button onclick={{action 'addCategory' newCategory}}>
  Add Category (rwj example)
</button>

twiddle.json

{
  "version": "0.10.7",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.0",
    "ember-data": "2.10.0",
    "ember-template-compiler": "2.10.0",
    "ember-testing": "2.10.0"
  },
  "addons": {}
}