Composing `friendships` from the constituent parts - doesn't work

person.js

export default Model.extend({
  name: DS.attr(),
  frienderFriendships: DS.hasMany('friendship', { inverse: 'friender' }),
  friendedFriendships: DS.hasMany('friendship', { inverse: 'friended' }),

  friendships: Ember.computed('frienderFriendships', 'friendedFriendships', function() {
    let friendedFriendshipsRel = this.hasMany('friendedFriendships');
    let frienderFriendshipsRel = this.hasMany('frienderFriendships');

    let frienderFriendships;
    if (frienderFriendshipsRel.value() !== null) {
      frienderFriendships = this.get('frienderFriendships');
    } else {
      frienderFriendships = frienderFriendshipsRel.load();
    }

    let friendedFriendships;
    if (friendedFriendshipsRel.value() !== null) {
      friendedFriendships = this.get('friendedFriendships');
    } else {
      friendedFriendships = friendedFriendshipsRel.load();
    }

    return Ember.RSVP.all([frienderFriendships, friendedFriendships]).then(([frienders, friendeds]) => {
      return frienders.concat(friendeds);
    });
  })
});