[web2py] How to compute Field on update based on previous value.

[François Delpierre]

Hi,

Here is exactly what I would like to have, but it does not compute on 
update :

db.define_table('t_templates',
      Field('f_template', type='upload', label=T('File'),notnull=True),     
                                                                            
      Field('f_revision', type='integer', label=T('Revision'),
          default=1,
          compute=lambda row: row['f_revision']+1 ,
          ),
      )