gistfile1.rb

@incoming_items.each do |item|
  # find all pending orders with this item
  pending_shipments = Shipments.in_status('waiting to ship').with_item(item).sorted_by_date
  
  allocated_quantity = 0
  # ship the allocated
  pending_shipments.each do |shipment|
    quantity = shipment.line_items.find_all_by_item_id(item.id).inject(0) {|sum, line| sum += line }
    if quantity + allocated_quantity < item.quantity
      # ship this shipment
      allocated_quantity += quantity
    end
    last if allocated_quantity >= item.quantity
  end
end