group-by.js

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

const {
  A,
  Helper,
  computed,
  defineProperty,
  get,
  isArray,
  observer,
  set,
  isPresent
} = Ember;

const groupFunction = function() {
  const groups = new A();
  const items = get(this, 'array');
  const property = get(this, 'byPath');

  const promises = items.map(function(item) {
    const value = get(item, property);

    return Ember.RSVP.resolve(value).then((resolvedValue) => {
      let group = groups.findBy(property, resolvedValue);

      if (isPresent(group)) {
        get(group, 'items').push(item);
      } else {
        group = { [property]: resolvedValue, items: [item] };
        groups.push(group);
      }
    });
  });

  return DS.PromiseArray.create({
    promise: Ember.RSVP.all(promises).then(() => { return groups; })
  })
};

export default Helper.extend({
  compute([byPath, array]) {
    set(this, 'array', array);
    set(this, 'byPath', byPath);

    return get(this, 'content');
  },

  byPathDidChange: observer('byPath', function() {
    let byPath = get(this, 'byPath');

    if (byPath) {
      defineProperty(this, 'content', computed(`array.@each.${byPath}`, groupFunction));
    } else {
      defineProperty(this, 'content', null);
    }
  }),

  contentDidChange: observer('content', function() {
    this.recompute();
  })
});