你可以使用django-model-changes来做到这一点，而不需要额外的数据库查询:

from django.dispatch import receiver
from django_model_changes import ChangesMixin

class Alias(ChangesMixin, MyBaseModel):
   # your model

@receiver(pre_save, sender=Alias)
def do_something_if_changed(sender, instance, **kwargs):
    if 'remote_image' in instance.changes():
        # do something

这是Chris Pratt的答案的一个版本，通过使用事务块和select_for_update()来防止竞争条件，同时牺牲性能。

@receiver(pre_save, sender=MyModel)
@transaction.atomic
def do_something_if_changed(sender, instance, **kwargs):
    try:
        obj = sender.objects.select_for_update().get(pk=instance.pk)
    except sender.DoesNotExist:
        pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
    else:
        if not obj.some_field == instance.some_field: # Field has changed
            # do something

另一个迟来的答案，但如果你只是想看看一个新文件是否已经上传到一个文件字段，试试这个:(改编自Christopher Adams对链接http://zmsmith.com/2010/05/django-check-if-a-field-has-changed/的评论在zach的评论这里)

更新链接:https://web.archive.org/web/20130101010327/http://zmsmith.com:80/2010/05/django-check-if-a-field-has-changed/

def save(self, *args, **kw):
    from django.core.files.uploadedfile import UploadedFile
    if hasattr(self.image, 'file') and isinstance(self.image.file, UploadedFile) :
        # Handle FileFields as special cases, because the uploaded filename could be
        # the same as the filename that's already there even though there may
        # be different file contents.

        # if a file was just uploaded, the storage model with be UploadedFile
        # Do new file stuff here
        pass

有一个属性__dict__，它将所有字段作为键，并将值作为字段值。所以我们可以比较其中两个

只需将模型的save函数更改为下面的函数即可

def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
    if self.pk is not None:
        initial = A.objects.get(pk=self.pk)
        initial_json, final_json = initial.__dict__.copy(), self.__dict__.copy()
        initial_json.pop('_state'), final_json.pop('_state')
        only_changed_fields = {k: {'final_value': final_json[k], 'initial_value': initial_json[k]} for k in initial_json if final_json[k] != initial_json[k]}
        print(only_changed_fields)
    super(A, self).save(force_insert=False, force_update=False, using=None, update_fields=None)

使用示例:

class A(models.Model):
    name = models.CharField(max_length=200, null=True, blank=True)
    senior = models.CharField(choices=choices, max_length=3)
    timestamp = models.DateTimeField(null=True, blank=True)

    def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
        if self.pk is not None:
            initial = A.objects.get(pk=self.pk)
            initial_json, final_json = initial.__dict__.copy(), self.__dict__.copy()
            initial_json.pop('_state'), final_json.pop('_state')
            only_changed_fields = {k: {'final_value': final_json[k], 'initial_value': initial_json[k]} for k in initial_json if final_json[k] != initial_json[k]}
            print(only_changed_fields)
        super(A, self).save(force_insert=False, force_update=False, using=None, update_fields=None)

产生只包含已更改字段的输出

{'name': {'initial_value': '1234515', 'final_value': 'nim'}, 'senior': {'initial_value': 'no', 'final_value': 'yes'}}

虽然这实际上并没有回答你的问题，但我会以不同的方式来讨论这个问题。

成功保存本地副本后，只需清除remote_image字段。然后在保存方法中，只要remote_image不为空，就可以随时更新图像。

如果您想保留对url的引用，您可以使用一个不可编辑的布尔字段来处理缓存标志，而不是使用remote_image字段本身。

我有点晚了，但我也找到了这个解决方案: Django脏域