Symfony: Doctrine order by calculated field

Положим нам нужно вывести 10 наиболее комментируемых постов.
Кусок схемы:

Post:
  tableName: blog_post
  actAs:
    Timestampable: ~
    Taggable: ~
  columns:
    id:           { type: integer(4), primary: true, autoincrement: true }
    title:        { type: string(255) }
    extract:      { type: string(1024) }
    content:      { type: string(4096) }
    is_published: { type: boolean, default: false }

Comment:
  tableName: blog_comment
  actAs:
    Timestampable: ~
  columns:
    id:           { type: integer(4), primary: true, autoincrement: true }
    post_id:      { type: integer(4), notnull: true }
    name:         { type: string(100) }
    email:        { type: string(100) }
    content:      { type: string(4096) }
    subscribe:    { type: boolean, default: false }
  relations:
    Post:
      class: Post
      local: post_id
      foreign: id
      foreignAlias: PostComments
      type: one
      foreignType: many
      onDelete: CASCADE

Получим популярные посты одним запросом:

  public function executeGetMostCommentedBlogPosts()
  {
    $q = Doctrine_Query::create()
      ->select('p.*')
      ->addSelect('(SELECT count(*) FROM PostComments pc WHERE pc.post_id = p.id) as comments_count')
      ->from('Post p')
      ->where('p.is_published', true)
      ->orderBy('comments_count DESC')
      ->addOrderBy('d.created_at DESC')
      ->limit(10);

    $this->posts = $q->execute();
  }