Someone

    $q = new Doctrine_Query();
    $q->select('u.*, COUNT(b.id) post_count')
      ->from('User u')
      ->leftjoin('u.Blog b')
      ->where('u.username = ?')
      ->groupby('u.id');
    $users = $q->execute(array($username));
    $count =  $users[0]['Blog'][0]['post_count'];