Django 自定义保存模型

我正在尝试在 django 的保存模型中执行基本日期计算，请参见下面的代码：

  class Purchase(models.Model):
    purchase_date = models.DateField()
    purchase_place = models.CharField(verbose_name='Place of Purchase', max_length=255)
    purchaseCategory = models.ForeignKey(PurchaseCategory, verbose_name='Purchase Category')
    cost = models.DecimalField(max_digits=11, decimal_places=2)
    warranty_period_number = models.IntegerField()
    warranty_period_type = models.CharField(max_length=255, choices=(('m', 'Month(s)'), ('y', 'Year(s)')))
    warranty_end_date = models.DateField(editable=False)
    scan = models.CharField(max_length=255)
    alerts = models.BooleanField(verbose_name='Receive Email Alerts?')
    user = models.ForeignKey('auth.User', editable=False)
    created = models.DateTimeField(editable=False, auto_now_add=True)
    modified = models.DateTimeField(editable=False, auto_now=True)

    #custom save model
    def save(self, *args, **kwargs):
        #figure out warranty end date
        if self.warranty_period_type == 'm':
            self.warranty_end_date = self.purchase_date + self.purchase_date.timedelta(months=self.warranty_period_number)
        else:
            self.warranty_end_date = self.purchase_date + self.purchase_date.timedelta(years=self.warranty_period_number)
        super(Purchase, self).save()

我认为以下内容会起作用，但没有运气..它出错了：

'datetime.date' object has no attribute 'timedelta'

任何人都可以指出正确的方向来做我想做的事情吗？

干杯， 本

timedelta 不接受年份，所以

from datetime import timedelta
# custom save model
def save(self, *args, **kwargs):
    # figure out warranty end date
    if self.warranty_period_type == 'm':
        self.warranty_end_date = self.purchase_date + timedelta(days=self.warranty_period_number*31)
    else:
        self.warranty_end_date = self.purchase_date + timedelta(days=self.warranty_period_number*365.2425)
    super(Purchase, self).save(*args, **kwargs)

Refs Python 时间增量（以年为单位）