[web2py] Model file variable scope

[NetAdmin]

in gluon/tools.py, I'm trying to modify the register() function
to save request.vars.get('last_name', None ) to a table defined in
db.py as
db.define_table('tbextra',
  SQLField('lastn', 'string', length = 15 )
)
but tools.py does not recognize the db part of  db.tbextra.lastn
I thought variables in the model files were global.

What am I doing wrong?

Thanks