Yeah! I fixed it. :D I'll answer my own question because there is an up vote..
It was pretty easy, just use Fluent on the returned object like this:
public function items($user_id = 0) {
$items = $this->has_many('Item');
// do we need to get the order?
if($user_id != 0) {
$items = $items->left_join('user_item_order', 'items.id', '=', 'user_item_order.item_id')
->where('user_item_order.user_id', '=', $user_id)
->order_by('user_item_order.order', 'asc');
}
return $items->get();
}