Group-By Feature Reques

controllers.application.js

import Ember from 'ember';

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

models.artist.js

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

export default Model.extend({
  name: attr('string'),
  category: attr('string'),
});

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.store.push({
      data: [
        {
          id: '1',
          type: 'artist',
          attributes: {
            name: 'Dead Prez',
            category: 'Hip Hop'
          }
        },
        {
          id: '2',
          type: 'artist',
          attributes: {
            name: 'Philip Glass',
            category: 'Classical'
          }
        },
        {
          id: '3',
          type: 'artist',
          attributes: {
            name: 'Claude DeBussy',
            category: 'Classical'
          }
        },
        // Uncommenting this breaks group-by
        //{
        //  id: '4',
        //  type: 'artist',
        //  attributes: {
        //    name: 'The Flaming Lips'
        //  }
        //}
      ]
    });
  }
});

templates.application.hbs

<h1>Ember Composable Helpers Group-By Bug / Feature Request</h1>

<a href='https://github.com/DockYard/ember-composable-helpers/issues/223' target='_blank'>https://github.com/DockYard/ember-composable-helpers/issues/223</a>

{{outlet}}

templates.index.hbs

<h1>Example</h1>
{{#each-in (group-by "category" model) as |category artists|}}
  <h1>{{category}} Artists</h1>
  
  <ul>
    {{#each artists as |artist|}}
      <li>{{artist.name}}</li>
    {{/each}}
  </ul>
{{/each-in}}

<hr>

<h1>Suggestion for group-by helper</h1>
<p>
Ideally, we'd be able to have a group for unknown artist types. In the index route, if you uncomment the last artist in the model (The Flaming Lips), you'd get this exception:

<pre>
Uncaught Error: Assertion Failed:
The key provided to get must be a string, you passed undefined
</pre>

thrown <a href='https://github.com/DockYard/ember-composable-helpers/blob/f7a913dfc7a7545083b56419c4b98709fb00f5a7/addon/helpers/group-by.js#L21' target='_blank'>Here</a>
</p>

<p>
 It'd be nice if we could say something like:
 
<pre>
(group-by "category" model 'Unknown Category')
</pre>

to handle this and not throw an exception.
</p>

twiddle.json

{
  "version": "0.10.4",
  "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-data": "2.6.0",
    "ember-template-compiler": "2.6.0"
  },
  "addons": {
  	"ember-composable-helpers": "1.1.1"
  }
}