[web2py] db field date time compare validation

[黄祥]

hi,

is it possible to compare db field datetime during form validation?
e.g.
db.define_table('duration',
                Field('from',
                      'datetime',
                      requires = IS_DATE()
                      ),
                Field('to',
                      'datetime',
                      requires = [IS_DATE(),
                                  request.vars.from < request.vars.to]
                      )
                )

an error occured said:
TypeError: 'bool' object is not callable

also i tried:

db.define_table('duration',
                Field('from',
                      'datetime',
                      requires = IS_DATE()
                      ),
                Field('to',
                      'datetime',
                      requires = [IS_DATE(),
                                  form.vars.from < form.vars.to]
                      )
                )

an error occured said :
NameError: name 'form' is not defined

does anyone have a clue to solve it?

thank you very much for your kind attention