OpenERP Python对象＆＃34;字段date_start在browse_record中找不到（hr.contract，56）＆＃34;

Question

我收到此错误：AttributeError: 'Field date_start not found in browse_record(hr.contract, 57)'

但表date_start中确实存在字段hr.contract。这是代码的一部分：

def _iep_calc(self, cr, uid, ids, field, arg, context=None):
    res = {}
    for payslip in self.browse(cr, uid, ids, context=context):
        contrat = self.pool.get('hr.contract')
        for contrat_date_from in contrat.browse(cr, uid, ids, context):
            date_f = contrat_date_from.date_start ## here is the problem
            date_t = datetime.date.today()
            DATETIME_FORMAT = "%Y-%m-%d"
            from_dt = datetime.datetime.strptime(str(date_f), DATETIME_FORMAT)
            to_dt = datetime.datetime.strptime(str(date_t), DATETIME_FORMAT)
            timedelta = to_dt - from_dt
            diff_day = (timedelta.days + float(timedelta.seconds) / 86400) / 365
            res[payslip.id] = diff_day
    return res