Writing an ActiveRecord scope with a join and "OR" condition

I have two tables: shipments and manifests.

• shipments: id, name
• manifests: id, shipment_id, start_time, end_time

A Shipment has_many Manifests, and a Manifest belongs_to one Shipment.

How do I write an ActiveRecord scope that returns me all the Shipments where:

1. if it has at least 开发者_开发技巧one manifest, the current time is between the start and end times, OR
2. it has no manifests

Hi I'm not sure if it can be achieved through scopes in ruby it may be sth like

   shipments = Shipments.all( :include => :manifests )
   empty_shipments = shipments.select { |item| item.manifests.blank? }
   non_empty_shipments = shipments - empty_shipments
   non_empty_shipments.delete_if {|item| (item.t_start..item.t_end).cover? Time.now}
   empty_shipments & non_empty_shipments