Django：保存时，如何检查字段是否已更改？ [英] Django: When saving, how can you check if a field has changed?

问题描述

在我的模型中，我有：

class Alias(MyBaseModel):
    remote_image = models.URLField(max_length=500, null=True, help_text="A URL that is downloaded and cached for the image. Only
 used when the alias is made")
    image = models.ImageField(upload_to='alias', default='alias-default.png', help_text="An image representing the alias")


    def save(self, *args, **kw):
        if (not self.image or self.image.name == 'alias-default.png') and self.remote_image :
            try :
                data = utils.fetch(self.remote_image)
                image = StringIO.StringIO(data)
                image = Image.open(image)
                buf = StringIO.StringIO()
                image.save(buf, format='PNG')
                self.image.save(hashlib.md5(self.string_id).hexdigest() + ".png", ContentFile(buf.getvalue()))
            except IOError :
                pass

哪些功能第一次很好， remote_image 更改。

Which works great for the first time the remote_image changes.

如果有人修改了别名的 remote_image ，我如何获取新图像？其次，是否有更好的方法来缓存远程图像？

How can I fetch a new image when someone has modified the remote_image on the alias? And secondly, is there a better way to cache a remote image?

推荐答案

虽然有点迟了，解决这个问题的其他人。实质上，您要覆盖 __ init __ 方法 models.Model ，以便保留原始值的副本。这样做就不用再做一个DB查找（这总是一件好事）。

Though it's a bit late, let me throw out this solution for others that come across this post. Essentially, you want to override the __init__ method of models.Model so that you keep a copy of the original value. This makes it so that you don't have to do another DB lookup (which is always a good thing).

class Person(models.Model):
  name = models.CharField()

  __original_name = None

  def __init__(self, *args, **kwargs):
    super(Person, self).__init__(*args, **kwargs)
    self.__original_name = self.name

  def save(self, force_insert=False, force_update=False, *args, **kwargs):
    if self.name != self.__original_name:
      # name changed - do something here

    super(Person, self).save(force_insert, force_update, *args, **kwargs)
    self.__original_name = self.name

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