/** @MongoDBDocument */
class Users {

    /** @MongoDBReferenceMany(targetDocument="Posts", mappedBy="user") */
    private $posts;

}

/** @MongoDBDocument */
class Posts {

    /** @MongoDBId */
    private $wallPostId;

    /** @MongoDBReferenceOne(targetDocument="Users", inversedBy="posts") */
    private $user;

    /** @MongoDBReferenceMany(targetDocument="Comments", mappedBy="post") */
    private $comments;

    /** @MongoDBString */
    private $content;

}

/** @MongoDBDocument */
class Comments {

    /** @MongoDBId */
    private $commentId;

    /** @MongoDBReferenceOne(targetDocument="Posts", inversedBy="comments") */
    private $post;

    /** @MongoDBString */
    private $content;

}

$post = $wallPostsAction->getWallPostById("5051d2a1e71a382c1b000000");
$output = "";
$output .= "Wall post content: " . $post->getContent() . "<br>";
//      $comment = new Comments();
//      $comment->setContent("Nice topic!");
//      $comment->setPost($post);
//      $this->dm->persist($comment);
//      $this->dm->flush();

echo count($post->getComments());
foreach($post->getComments() as $comment){
    $output .= " comment: " . $comment->getComment() . "<br>";
}

{
   "_id": ObjectId("---"),
   "post"?: {
     "$ref": "Posts",
     "$id": ObjectId("---"),
     "$db": "db"
  },
   "content": "Nice topic!"
}

->field('comments')->prime(true)