Getting a different model from an association

gistfile1.rb

class Shipment
  has_many :shipping_items, :inverse_of => :shipment, :dependent => :destroy do
    def linked_po_ids
      joins(:purchased_item => :purchase_order).select('DISTINCT purchase_orders.id AS po_id').map{|si| si.po_id}
    def linked_pos
      ? #how to return the purchase orders (objects) ?
    end
  end

def linked_pos
   PurchaseOrder.joins(:purchased_items => :shipping_items).where(:shipping_items => {:shipment_id => self.id}).select('DISTINCT purchase_orders.*')   
end

end