[web2py] Referencing the membership table in custom user model.

[Luther Goh Lu Feng]

I have the following custom user model. I am trying to reference the membership 
type so that when users are created by administrators, they can assigned 
different roles. Attention to be drawn to the 2 lines commented out at the 
bottom, which I have tried to make the reference but failed. Any tips?

db.define_table(auth.settings.table_user_name, 
    Field('username', 'string', length=255, required=True, 
requires=IS_NOT_EMPTY(), unique=True, label="Username"),
    Field('first_name', 'string', length=255, required=True, 
requires=IS_NOT_EMPTY(), label="First name"),
    Field('last_name', 'string', length=255, required=True, 
requires=IS_NOT_EMPTY(), label="Last name"),
    Field('password', 'password', length=40, required=True, 
requires=[IS_NOT_EMPTY(),CRYPT(key="sha1:theultimatesuperrover")], 
 readable=False,  label="Password"),
    Field('last_login', 'datetime', writable=False, readable=False, 
update=request.now),
    Field('registration_key', length=512, writable=False, readable=False, 
default=''),
    Field('reset_password_key', length=512, writable=False, readable=False, 
default=''),
    Field('registration_id', length=512, writable=False, readable=False, 
default=''),
    Field('class_id', 'reference classes', default=''),
    #Field('membership_id', db.auth_membership, default=''),
    #Field('membership_id', auth.settings.table_membership, default=''),
    format='%(username)s'
)