I am tryint to extend stock.picking class with a new method:

def download_item_labels(self,id, **kw):

  Model = request.registry['stock.picking']

  cr, uid, context = request.cr, request.uid, request.context

  fields = ['id']

  res = Model.read(cr, uid, [int(id)], fields, context)[0]

How can I get all product codes (I guess that products i stock.picking are not linked to product) included in that stock.picking?