symfonyのCriteriaでOrder by
$c->addAscendingOrderByColumn(CommentPeer::CREATED_AT);
$c->addDescendingOrderByColumn(CommentPeer::CREATED_AT);
$c->addAscendingOrderByColumn(CommentPeer::CREATED_AT);
$c->addDescendingOrderByColumn(CommentPeer::CREATED_AT);