there are two ways:
db.define_table('quantity',
Field('value','integer'))
db.define_table('item',
Field('unit_price','double'),
Field('quantity',db.quantity),
Field('total_price',
compute=lambda r:
r['unit_price']*db.quantity[r.quantity].value))
or
compute=lambda r: r['unit_price']
*db.quantity(r['quantity']).value
