[web2py] help on a query

[Manuele Pesenti]

defining a table like that

db.define_table('power_curve',
    Field('site', db.site, notnull=True, requires = IS_IN_DB(db,
        'site.id', '%(name)s')),
    Field('file_name', 'upload', uploadfield=True, notnull=True,
        required=True),
    Field('validity', 'date', default=datetime.date(1970, 1, 1), 
requires=IS_EMPTY_OR(IS_DATE(format='%d/%m/%Y'))
)
how can I select for each site id the last file name inserted?
I tryied this two query but they give the same result and they returns 
the first file name in each case:

db(db.power_curve).select(db.power_curve.site
        db.power_curve.file_name,
        db.power_curve.validity,
        groupby=db.power_curve.site,
    orderby=db.power_curve.site|~db.power_curve.start)

db(db.power_curve).select(db.power_curve.site
        db.power_curve.file_name,
        db.power_curve.validity,
        groupby=db.power_curve.site,
    orderby=db.power_curve.site|db.power_curve.start)

thank you

        Manuele