Ember.js computed property averaging value on `hasMany` Firebase relationship in model.

model.js

//this is the model for an object that has many reviews. reviews each contain a score. the averageScore property below gathers
// all reviews, sums all scores from all reviews, and divides by the number of reviews to calculate an average score.

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

export default DS.Model.extend({
  reviews: DS.hasMany('review', {async: true}),
  ...
  averageScore: Ember.computed('[email protected]', function() {
    return this.get('reviews').reduce(function(sum, review) {
      return sum += review.get('rating');
    }, 0) / this.get('reviews').get('length');
  })
});