我对@iperelivskiy的解决方案的看法是:在大规模的情况下，为每个__init__创建_initial字典是昂贵的，而且大多数时候是不必要的。我稍微改变了mixin，这样它只在你显式地告诉它这样做(通过调用instance.track_changes)时才记录更改:

from typing import KeysView, Optional
from django.forms import model_to_dict

class TrackChangesMixin:
    _snapshot: Optional[dict] = None

    def track_changes(self):
        self._snapshot = self.as_dict

    @property
    def diff(self) -> dict:
        if self._snapshot is None:
            raise ValueError("track_changes wasn't called, can't determine diff.")
        d1 = self._snapshot
        d2 = self.as_dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if str(v) != str(d2[k])]
        return dict(diffs)

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

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

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

作为SmileyChris回答的扩展，您可以为last_updated的模型添加一个datetime字段，并在检查更改之前为您将让它达到的最大年龄设置某种限制

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。

这是另一种方法。

class Parameter(models.Model):

    def __init__(self, *args, **kwargs):
        super(Parameter, self).__init__(*args, **kwargs)
        self.__original_value = self.value

    def clean(self,*args,**kwargs):
        if self.__original_value == self.value:
            print("igual")
        else:
            print("distinto")

    def save(self,*args,**kwargs):
        self.full_clean()
        return super(Parameter, self).save(*args, **kwargs)
        self.__original_value = self.value

    key = models.CharField(max_length=24, db_index=True, unique=True)
    value = models.CharField(max_length=128)

根据文档:验证对象

full_clean()执行的第二步是调用Model.clean()。应该重写此方法，以便在模型上执行自定义验证。 此方法应用于提供自定义模型验证，并根据需要修改模型上的属性。例如，你可以使用它来自动为一个字段提供一个值，或者进行验证，需要访问多个字段:

现在直接回答:检查字段的值是否已经改变的一种方法是在保存实例之前从数据库获取原始数据。想想这个例子:

class MyModel(models.Model):
    f1 = models.CharField(max_length=1)

    def save(self, *args, **kw):
        if self.pk is not None:
            orig = MyModel.objects.get(pk=self.pk)
            if orig.f1 != self.f1:
                print 'f1 changed'
        super(MyModel, self).save(*args, **kw)

同样的事情也适用于处理表单。您可以在ModelForm的clean或save方法中检测到它:

class MyModelForm(forms.ModelForm):

    def clean(self):
        cleaned_data = super(ProjectForm, self).clean()
        #if self.has_changed():  # new instance or existing updated (form has data to save)
        if self.instance.pk is not None:  # new instance only
            if self.instance.f1 != cleaned_data['f1']:
                print 'f1 changed'
        return cleaned_data

    class Meta:
        model = MyModel
        exclude = []

我扩展了@livskiy的mixin，如下所示:

class ModelDiffMixin(models.Model):
    """
    A model mixin that tracks model fields' values and provide some useful api
    to know what fields have been changed.
    """
    _dict = DictField(editable=False)
    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.
        """
        object_dict = model_to_dict(self,
               fields=[field.name for field in self._meta.fields])
        for field in object_dict:
            # for FileFields
            if issubclass(object_dict[field].__class__, FieldFile):
                try:
                    object_dict[field] = object_dict[field].path
                except :
                    object_dict[field] = object_dict[field].name

            # TODO: add other non-serializable field types
        self._dict = object_dict
        super(ModelDiffMixin, self).save(*args, **kwargs)

    class Meta:
        abstract = True

DictField为:

class DictField(models.TextField):
    __metaclass__ = models.SubfieldBase
    description = "Stores a python dict"

    def __init__(self, *args, **kwargs):
        super(DictField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        if not value:
            value = {}

        if isinstance(value, dict):
            return value

        return json.loads(value)

    def get_prep_value(self, value):
        if value is None:
            return value
        return json.dumps(value)

    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_db_prep_value(value)

可以通过在模型中扩展它来使用它 一个_dict字段将在同步/迁移时添加，该字段将存储对象的状态