[web2py] chained option box

[kibrun]

Dear all,

     I'm new in python and currently trying web2py. I need some help
to build a chained option box - meaning when i choose an item in
option1's list, option2's list automatically change to a specific list
that falls under option1's subject:

db.define_table('spent',
                SQLField('spent'))

db.define_table('spenton',
                SQLField('herespent'),
                SQLField('spent_id', db.spent))
db.spenton.spent_id.requires=IS_IN_DB(db, db.spent.id, '%(spent)s')

if i choose an entry from option list of db.spent, the option list
under db.spenton.herespent will change according to the respective
spenton.spent_id without displaying the rest of the entries in
'herespent'

I need to know what is the simplest way to do it in web2py? thanks in
advance.