保存时，如何检查字段是否已更改？ [英] 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

remote_image 的第一次更改效果很好。

Which works great for the first time the remote_image changes.

当有人修改了别名上的 remote_image 时，如何获取新图像？其次，还有更好的方法来缓存远程图像吗？

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

基本上，您想覆盖<$ c $ models.Model 的c> __ init __ 方法，以便保留原始值的副本。这样一来，您不必进行其他数据库查找（这总是一件好事）。

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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