def _rut(self, cr, uid, ids, field_name, args, context=None):
    res = {}
    for i in self.browse(cr, uid, ids, context):
        res[i.id] = ''
        if i.partner_id:
            res[i.id] = i.partner_id.vat
    return res

_columns = {
    'rut': fields.function(_rut, type='char', string='Rut')
}