DMZ 1.7.1 (DataMapper OverZealous Edition)

[eluser]Wazzu[/eluser]
Order by foreing table field
Hi all. I have this two tables:
- products (id, name, category_id)
- categories (id, name)

So I make a list of products this way:

Code:

$p = new Product();
$p->order_by(category_id);
$p->get();
foreach($p as $product) {
  $product->category->get();
  echo $product->category->name . " " . $product->name . "<br />"
}

That way, my list gets ordered by category ID, but not by category name.
How should I do it?