Find all objects with no associated has_many objects

Question

In my online store, an order is ready to ship if it in the "authorized" state and doesn't already have any associated shipments. Right now I'm doing this:

class Order < ActiveRecord::Base
    has_many :shipments, :dependent => :destroy

    def self.ready_to_ship
        unshipped_orders = Array.new
        Order.all(:conditions => 'state = "authorized"', :include => :shipments).each do |o|
            unshipped_orders << o if o.shipments.empty?
        end
        unshipped_orders
    end
end

Is there a better way?

Answer 1

In Rails 3 using AREL

Order.includes('shipments').where(['orders.state = ?', 'authorized']).where('shipments.id IS NULL')