相关疑难解决方法(0)

我正在遵循本指南，但在保存时出现以下错误...

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

为什么我会收到这个错误？我在下面包含了完整的模型。

谢谢

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 = …