Flatten CP

controllers.application.js

import Ember from 'ember';

const A = Ember.A;
const O = Ember.Object.create.bind(Ember.Object);

const getCpO = function(data) {
	return Ember.Object.extend({
    childKey: Ember.computed('children.[]', function(){
    	return (this.get('children')||A()).map(e=>e.toString()).join('-');
    })
  }).create(data);
}

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  parents: A([
    getCpO({ children: [ O({name: "Alpha", id: 1}), O({name: "Bravo"}) ] }),
    getCpO({ children: [ O({name: "Charlie"}) ] }),
  ]),
  
  // First, we need an array of arrays
  childArrays: Ember.computed("parents.@each.childKey", function(){
    console.log('childKey', this.get('parents').mapBy('childKey'));
  	return this.get('parents').mapBy('children');
  }),
  
  // Then we flatten it
  children: Ember.computed("childArrays.[]", function () {
		return this
    	.get("childArrays")
    	.reduce((a, b) => a.concat(b), A());
  }),
  
  actions: {
    addChild () {
      this
      	.get("parents.lastObject.children")
      	.pushObject(O({ name: Math.random().toString(36) }));
    }
  }
});
 

templates.application.hbs

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

<h2>Parents:</h2>

{{#each parents as |parent i|}}
  <p>
  	{{i}}: 
    {{#each parent.children as |child|}}
      {{child.name}}
    {{/each}}  
  </p>
{{/each}}

<h2>All children:</h2>
<p>
  {{#each children as |child|}}
    {{child.name}}
  {{/each}}
</p>

<p>
	<button {{action "addChild"}}>Add child</button>
</p>

twiddle.json

{
  "version": "0.13.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}