Ordering across related tables

I have two related tables: groups and categories. In the models,

   Group
      has_many "categories"

   Category
      belongs_to :group

Category does have a column, "group_id" as required.
Both the groups and categories tables have a "sortorder" column which
dictates the order the items are to be presented.

In my controller, I want to retrieve the Categories along with the
information on the Group it belongs to. They should be retrieved in
sortorder sequence with group having the highest precedence.

How can I specify a find statement with an :order_by that specifies both
Group.sortorder and Category.sortorder?

Group.find(:all, :include=>:categories, :order=>'groups.sortorder,
categories.sortorder')