[web2py] recursive sum in virtual fields

[Serbitar]

Consider:

db.define_table('item',
    Field('unit_price','double'))

db.define_table('order_item',
    Field('item',db.item),
    Field('quantity','integer'))


I want to create a virtual field item_count, in item, that is the sum
of the entries in order_item with order_item.item = item.id

I tried ( very basic summing routine to not introduce any errors
here):

class MyVirtualFields(object):
    def item_countself):
        sum = 0
        rows = self.item.order_item.select()
            for row in rows:
                sum += row.quantity
        return sum

db.item.virtualfields.append(MyVirtualFields())


RuntimeError: maximum recursion depth exceeded


Any ideas?