web2py How to create computed field in DB -

i going adding fields table schema in web2py using db.py file...

i want add 3 new fields, 2 datetime , 1 field show timespan of other 2 fields...

q: how can define 3rd field automatically populated difference of 2 datetime values row?

try following:

>>> db.define_table('item',                 field('unit_price','double'),                 field('quantity','integer'),                 field('total_price',                       compute=lambda r: r['unit_price'] * r['quantity']))  >>> r = db.item.insert(unit_price=1.99, quantity=5) >>> print r.total_price 9.95 

see http://web2py.com/books/default/chapter/29/06/the-database-abstraction-layer#computed-fields