[web2py] add foreign key to auth_user

[Alex]

Hi,
I have a simple question : I would like to add a foreign key to the
table auth_user, but I don't know how to do that.
I can add a text field easily. For example :

auth = Auth(db)
auth.settings.extra_fields['auth_user'] = [Field('city')]
auth.define_tables()

But if I do this, it doesn't work:

auth = Auth(db)
auth.settings.extra_fields['auth_user'] = [Field('myFK_id',
db.tableX)]
auth.define_tables()

db.define_table('tableX', .....)

I can't find anything on the Web...