[web2py] need help with db table

[pk]

hi,
i know that i can check one field with IS_NOT_IN_DB()
but i need the two fields which deposit not together in one row,
in my case 'hauptperson' and 'freund'.

here is my table:

#########################################################################
## Tabelle Freunde
#########################################################################
db.define_table('friends',
                SQLField('userid',
                         default=auth.user.id if auth.user else 0,
                         writable=False, readable=False),
                SQLField('hauptperson',
                         default=auth.user.user_name if auth.user else
0,
                         writable=False, readable=False,
label='Hauptperson'),
                SQLField('freund', label='Freund', unique=True),
                SQLField('freundseit', 'datetime',
default=request.now, label='Freund seit'),
                SQLField('bestaetigt','boolean', default=False))

db.friends.userid.requires=IS_IN_DB(db,'auth_user.id')
db.friends.hauptperson.requires=IS_IN_DB(db,'auth_user.user_name')
db.friends.freund.requires=IS_IN_DB(db,'auth_user.user_name')


can somebody help me???
thanks