Django模型错误超出最大递归深度

我正在关注这个guide http://www.acedevs.com/blog/2011/07/25/quick-qr-codes-django/保存时出现以下错误...

RuntimeError at /admin/products/product/2/
maximum recursion depth exceeded

为什么我会收到此错误？我在下面包含了完整的模型。

Thanks

模型.py：

class Product(models.Model):
    title = models.CharField(max_length=60)
    qr_url = models.URLField(blank=True)
    qr_image = models.ImageField(
        upload_to="public/uploads/",
        height_field="qr_image_height",
        width_field="qr_image_width",
        null=True,
        blank=True,
        editable=False
    )
    qr_image_height = models.PositiveIntegerField(null=True, blank=True, editable=False)
    qr_image_width = models.PositiveIntegerField(null=True, blank=True, editable=False)

    #FK
    category = models.ManyToManyField(ProductCategory)
    attribute_answers = models.ManyToManyField(AttributeAnswers)
    # Custom Managers

    def __unicode__(self):
        return self.title

    def qr_code(self):
        return '' % self.qr_image.url
    qr_code.allow_tags = True

def product_pre_save(sender, instance, **kwargs):
    if not instance.pk:
        instance._QRCODE = True
    else:
        if hasattr(instance, '_QRCODE'):
            instance._QRCODE = False
        else:
            instance._QRCODE = True


models.signals.pre_save.connect(product_pre_save, sender=Product)


def product_post_save(sender, instance, **kwargs):
    if instance._QRCODE:
        instance._QRCODE = False
    if instance.qr_image:
        instance.qr_image.delete()
    qr = QRCode(4, QRErrorCorrectLevel.L)
    qr.addData(instance.qr_url)
    qr.make()
    image = qr.makeImage()

    #Save image to string buffer
    image_buffer = StringIO()
    image.save(image_buffer, format='JPEG')
    image_buffer.seek(0)

    #Here we use django file storage system to save the image.
    file_name = 'UrlQR_%s.jpg' % instance.id
    file_object = File(image_buffer, file_name)
    content_file = ContentFile(file_object.read())
    instance.qr_image.save(file_name, content_file, save=True)


models.signals.post_save.connect(product_post_save, sender=Product)

您的后保存信号调用 save ，它调用后保存信号，该信号调用 save ...

这基本上是一个无限循环，运行时会因最大递归错误而终止。