Django modelformset_factory更新现有图像

Question

class ProductImageForm(forms.ModelForm):
    class Meta:
        model = ProductImage
        fields = ['product', 'original', 'caption']
        # use ImageInput widget to create HTML displaying the
        # actual uploaded image and providing the upload dialog
        # when clicking on the actual image.
        widgets = {
            'original': ImageInput(),
        }

    def save(self, *args, **kwargs):
        # We infer the display order of the image based on the order of the
        # image fields within the formset.
        kwargs['commit'] = False
        obj = super(ProductImageForm, self).save(*args, **kwargs)
        obj.display_order = self.get_display_order()
        obj.save()
        return obj

    def get_display_order(self):
        if self.prefix:
            return self.prefix.split('-').pop()
        else:
            return 1

我正在使用django modelformset_factory将图像插入到数据库中并更新图像。现在的问题是在使用modelformset_factory时将图像插入数据库而不是更新。以下是我的代码。请帮我更新除了保存

            ImageFormSet = modelformset_factory(ProductImage,
                                     form=ProductImageForm, extra=0, can_delete=True)
            formset = ImageFormSet(self.request.POST or None, self.request.FILES or None, data)
            if formset.is_valid():
                try:
                    instances = formset.save(commit=False)
                    for instance in instances:
                        instance.save()
                except Exception as e:
                    pass
            else:
                err = formset.errors
                raise error_message(err)