Update Fulfillment with Order ID

gistfile1.rb

 f = ShopifyAPI::Fulfillment.find(:first, :params => {:order_id => order_id})  
 if f.nil?
   fulfillment = ShopifyAPI::Fulfillment.new({:order_id => order_id, :notify_customer => true, :tracking_number => "123456"})
   fulfillment.save
 else
   f.tracking_number = '123456'
   f.notify_customer = true
   f.save
 end      

Thank you, this gist was exactly what I was looking for.
I am however having an issue tweaking this to fulfill a single line item. When I pass a line items parameter hash like is suggested in the shopify api (http://api.shopify.com/fulfillment.html#create) (e.g. :line_items => { "id" => v } ) the api returns a 500 error. Are you familiar with the correct syntax?

Thanks,
Stewart Knapman.