Nested Dependencies

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  recordings: [{
  id: 'recording1',
  recordingParts: ['a', 'b'],
  }, {
  id: 'recording2',
  recordingParts: ['c', 'd'],
  }],
  
    recordingParts: Ember.computed('recordings.@each.recordingParts', function() {
      const recordingParts = this.get('recordings').map(recording => Ember.get(recording, 'recordingParts'));
      console.log("New RecParts", recordingParts);
      return recordingParts;
  }),
  
  flatRecordingParts: Ember.computed('recordingParts.[]', function() {
    console.log("new flat rec Parts");
    return this.get('recordingParts').toArray().reduce((a, b) => a.concat(b), []);
  }),
  
  interestingProperty: Ember.computed('flatRecordingParts', function() {
    console.log("update interesting stuff");
    // without getting flatRecordingParts, interesting Property is not
    // updated although I don't need it's actual value here here.
    let foobar = this.get('flatRecordingParts');
    return this.get('recordings');
  }),
  
  actions: {
    buttonClicked() {
      console.log("Click");
      const firstRecording = this.get('recordings')[0];
      Ember.set(firstRecording, 'recordingParts', ['e', 'f']);
    },
  },
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
Recordings: {{recordings}}<br>
{{!--flatRecordingParts: {{flatRecordingParts}}--}}
interestingStuff: {{interestingProperty}}

<br>

<button {{action "buttonClicked"}}>foobar</button>s

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": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}