[web2py] how to disable field access if not authorized

[weheh]

I'm using crud and want to disable access to certain fields if user is
not a member of the admin group. For instance, content can only be
approved by admin. I want to register the id of the user who approved
the content, too. So I'd like to say:
  if not auth.has_membership(auth.id_group('Admin'),auth.user.id):
    db.content.approved.writable=False
    db.content.signature.writable=False
  form = crud.update(db.content,...)

so that even if the view has this:
            {{=form.custom.widget.approved}}
            {{=form.custom.widget.signature}}

the fields shouldn't be fillable by an unauthorized user. Problem is,
this doesn't work and I get this ticket:

Traceback (most recent call last):
  File "C:\web2py\gluon\restricted.py", line 173, in restricted
    exec ccode in environment
  File "C:/web2py/applications/myapp/controllers/admin.py", line 187,
in <module>
  File "C:\web2py\gluon\globals.py", line 96, in <lambda>
    self._caller = lambda f: f()
  File "C:\web2py\gluon\tools.py", line 1873, in f
    return action(*a, **b)
  File "C:/web2py/applications/myapp/controllers/admin.py", line 59,
in marketing
    form = crud.update(db.content,request.args(2),message=T('Updated
content'))
  File "C:\web2py\gluon\tools.py", line 2270, in update
    keepvalues=self.settings.keepvalues):
  File "C:\web2py\gluon\sqlhtml.py", line 755, in accepts
    onvalidation,
  File "C:\web2py\gluon\html.py", line 1267, in accepts
    status = self._traverse(status)
  File "C:\web2py\gluon\html.py", line 449, in _traverse
    newstatus = c._traverse(status) and newstatus
  File "C:\web2py\gluon\html.py", line 449, in _traverse
    newstatus = c._traverse(status) and newstatus
  File "C:\web2py\gluon\html.py", line 449, in _traverse
    newstatus = c._traverse(status) and newstatus
  File "C:\web2py\gluon\html.py", line 449, in _traverse
    newstatus = c._traverse(status) and newstatus
TypeError: 'NoneType' object is not callable

Anybody know what's going on here?

-- 
You received this message because you are subscribed to the Google Groups 
"web2py-users" group.
To post to this group, send email to web...@googlegroups.com.
To unsubscribe from this group, send email to 
web2py+unsubscr...@googlegroups.com.
For more options, visit this group at 
http://groups.google.com/group/web2py?hl=en.