Association Computation

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  actions: {
    addWord: function() {
      var word = this.store.createRecord('word', {
      });
      
      this.get('model.pages.firstObject.words')
        .pushObject(word)
    }
  }
});

application.route.js

import Ember from 'ember';

export default Ember.Route.extend({
  model: function() {
    return this.store.createRecord('book', {
      title: 'War and Peace',
      pages: [
        this.store.createRecord('page')
      ]
    });
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<button {{action 'addWord'}}>Add Word</button>
<br>
Book:{{model.title}}
<br>
Word Count: {{model.words.length}}
<br>
<br>
{{#each model.words as |word|}}
  {{word}}
{{/each}}
<br>
<br>

models.book.js

import DS from 'ember-data';
import Ember from 'ember';

const {
  computed
} = Ember;

export default DS.Model.extend({
  title: DS.attr('string'),
  pages: DS.hasMany('page'),
  pageWords: computed.mapBy('pages', 'words'),
  words: computed.sum('pageWords') 
});

models.page.js

import DS from 'ember-data';

export default DS.Model.extend({
  number: DS.attr('number'),
  words: DS.hasMany('word')
});

models.word.js

import DS from 'ember-data';

export default DS.Model.extend({
  
});

twiddle.json

{
  "version": "0.4.16",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.1.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.1.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.1.0/ember-template-compiler.js"
  }
}