[web2py] Enforce roles in db

[petrasadi]

In my table a have a field like this:  Field('manager', 'reference
auth_user')
Is there a way to enforce in my db for this 'manager' to be part of
the "managers" group?
I have tried db.myTable.manager.requires_membership('managers'), but
of course this blew up.
Any ideas would be highly appreciated.