Django 保存方法在模型更改时与信号发生冲突

Question

我想以较小的分辨率保存后封面和相应的后封面缩略图。 最后用户还可以更改后封面。 为了完成正确的文件处理，我正在使用 model.signals。 这似乎工作得很好，直到我尝试更改 Post 对象的 postcover。

当我尝试使用更改的 postcover 保存 Post 对象时，出现以下错误：

 ValueError: The 'postcover' attribute has no file associated with it.

初始化创建和删除 Post 对象按预期工作。

模型.py

class Post(models.Model):
...
    postcover = models.ImageField(
        verbose_name="Post Cover",
        blank=True,
        null=True,
    )
    postcover_tn = models.ImageField(
        verbose_name="Post Cover Thumbnail",
        blank=True,
        null=True,
    )
...

def save(self, *args, **kwargs):
    super(Post, self).save(*args, **kwargs)
    if self.postcover:
        if os.path.exists(self.postcover.path):
            image = Image.open(self.postcover)
            outputIoStream = BytesIO()
            baseheight = 600
            hpercent = baseheight / image.size[1]
            wsize = int(image.size[0] * hpercent)
            imageTemproaryResized = image.resize((wsize, baseheight))
            imageTemproaryResized.save(outputIoStream, format='PNG')
            outputIoStream.seek(0)
            self.postcover = InMemoryUploadedFile(outputIoStream, 'ImageField',
                                                  "%s.png" % self.postcover.name.split('.')[0], 'image/png',
                                                  sys.getsizeof(outputIoStream), None)

            image = Image.open(self.postcover)
            outputIoStream = BytesIO()
            baseheight = 100
            hpercent = baseheight / image.size[1]
            wsize = int(image.size[0] * hpercent)
            imageTemproaryResized = image.resize((wsize, baseheight))
            imageTemproaryResized.save(outputIoStream, format='PNG')
            outputIoStream.seek(0)
            self.postcover_tn = InMemoryUploadedFile(outputIoStream, 'ImageField',
                                                  "%s.png" % self.postcover.name.split('.')[0], 'image/png',
                                                  sys.getsizeof(outputIoStream), None)
    elif self.postcover_tn:
        self.postcover_tn.delete()

    super(Post, self).save(*args, **kwargs)

信号.py

@receiver(models.signals.post_save, sender=Post)
def post_auto_delete_files_on_change(sender, instance, **kwargs):
    """
    Deletes old file from filesystem
    when corresponding object is updated
    with new file.
    """
    if not instance.pk:
        return False

    try:
        old_postcover = sender.objects.get(pk=instance.pk).postcover
        old_postcover_tn = sender.objects.get(pk=instance.pk).postcover_tn
    except sender.DoesNotExist:
        return False
    if not old_postcover:
        return False

    new_postcover = instance.postcover
    if not old_postcover == new_postcover:
        if os.path.isfile(old_postcover.path):
            os.remove(old_postcover.path)
            os.remove(old_postcover_tn.path)

我的 forms.py 也有一个 postcover delete 选项，这可能会影响这个过程：

        if self.instance.postcover:
            self.fields['remove_cover'].disabled = False
            self.fields['postcover'].label = mark_safe('Overwrite Cover:')
            self.fields['remove_cover'].label = mark_safe('Remove Cover:')
        else:
            self.fields['postcover'].label = mark_safe('Cover:')
            del self.fields['remove_cover']


    def save(self, commit=True):
        instance = super(PostForm, self).save(commit=False)
        if self.cleaned_data.get('remove_cover'):
            try:
                os.unlink(instance.postcover.path)
            except OSError:
                pass
            instance.postcover = None
        if self.cleaned_data.get('remove_attachment'):
            try:
                os.unlink(instance.postattachment.path)
            except OSError:
                pass
            instance.postattachment = None
        if commit:
            instance.save()
        return instance

Answer 1

您应该使用post_save信号。 由于您现在使用pre_save ，因此在将对象保存到 DB 之前调用了您的信号处理程序。 将您的代码更改为：

@receiver(models.signals.post_save, sender=Post)
def post_auto_delete_files_on_change(sender, instance, **kwargs):