从Django 1.8开始，就有了from_db方法，Serge提到过。事实上，Django文档中包含了这样一个特定的用例:

https://docs.djangoproject.com/en/dev/ref/models/instances/#customizing-model-loading

下面是一个示例，展示如何记录从数据库加载的字段的初始值

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

我使用以下mixin:

from django.forms.models import model_to_dict


class ModelDiffMixin(object):
    """
    A model mixin that tracks model fields' values and provide some useful api
    to know what fields have been changed.
    """

    def __init__(self, *args, **kwargs):
        super(ModelDiffMixin, self).__init__(*args, **kwargs)
        self.__initial = self._dict

    @property
    def diff(self):
        d1 = self.__initial
        d2 = self._dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if v != d2[k]]
        return dict(diffs)

    @property
    def has_changed(self):
        return bool(self.diff)

    @property
    def changed_fields(self):
        return self.diff.keys()

    def get_field_diff(self, field_name):
        """
        Returns a diff for field if it's changed and None otherwise.
        """
        return self.diff.get(field_name, None)

    def save(self, *args, **kwargs):
        """
        Saves model and set initial state.
        """
        super(ModelDiffMixin, self).save(*args, **kwargs)
        self.__initial = self._dict

    @property
    def _dict(self):
        return model_to_dict(self, fields=[field.name for field in
                             self._meta.fields])

用法:

>>> p = Place()
>>> p.has_changed
False
>>> p.changed_fields
[]
>>> p.rank = 42
>>> p.has_changed
True
>>> p.changed_fields
['rank']
>>> p.diff
{'rank': (0, 42)}
>>> p.categories = [1, 3, 5]
>>> p.diff
{'categories': (None, [1, 3, 5]), 'rank': (0, 42)}
>>> p.get_field_diff('categories')
(None, [1, 3, 5])
>>> p.get_field_diff('rank')
(0, 42)
>>>

Note

请注意，此解决方案仅在当前请求的上下文中工作良好。因此，它主要适用于简单的情况。在并发环境中，多个请求可以同时操作同一个模型实例，您肯定需要一种不同的方法。

这在Django 1.8中是适用的

def clean(self):
    if self.cleaned_data['name'] != self.initial['name']:
        # 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

用David Cramer的解决方案怎么样:

http://cramer.io/2010/12/06/tracking-changes-to-fields-in-django/

我曾经这样成功地使用过:

@track_data('name')
class Mode(models.Model):
    name = models.CharField(max_length=5)
    mode = models.CharField(max_length=5)

    def save(self, *args, **kwargs):
        if self.has_changed('name'):
            print 'name changed'

    # OR #

    @classmethod
    def post_save(cls, sender, instance, created, **kwargs):
        if instance.has_changed('name'):
            print "Hooray!"