Question

I am writing a small, forum-like application (in Symfony2) and I am running into the following problem:

I have two Entities:

class ForumThread {
  public $fourmThreadId;
  public $title;
  public $posts; // aggregation field -> ArrayCollection of ForumPost (can't be empty)
}

class ForumPost {
  public $content;
  public $timeCreated;
  public $thread; // aggregation field -> ForumThread (can't be null)
}

I now want to get all forum threads, including only the newest post of each thread. I want to use it for the forum overview:

-------------------------------------
| Thread                | Last Post |
-------------------------------------
| Name of Thread #1     |    ###    |
| Name of Thread #2     |    ###    |
| ...                   |    ...    |
-------------------------------------

I want to fetch the complete ForumPost entity, not just a single field (like the timestamp).

Also it should be performant: ideally in 1-2 queries (I have found something in raw SQL using INNER JOINs, but I want in DQL or ideally using the QueryBuilder – and my translation tries from SQL to DQL did not succeed).

Answer 1

Ok, I found a solution, which works but is not be the best way:

$where = "fp.timeCreated = (SELECT MAX(fpi.timeCreated) FROM ApfelboxGuildBundle:ForumPost fpi WHERE fpi.thread = ft)";

$query = $this->getRepository()->createQueryBuilder("ft")
        ->select("ft, fp")
        ->join("ft.posts", "fp")
        ->groupBy("ft.forumThreadId")
        ->addOrderBy("ft.isSticky", "desc")
        ->addOrderBy("fp.timeCreated", "desc")
        ->addOrderBy("ft.forumThreadId", "desc")
        ->setFirstResult($offset)
        ->setMaxResults(self::THREADS_PER_PAGE);

Notice the subquery in the WHERE clause. This works, but has a flaw: it joins using the timestamp field (to prevent the additional nested subquery – which did always produce parsing errors in DQL), which may not be unique, even for the given thread.