保存时，如何检查字段是否已更改?

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

这在remote_image第一次改变的时候工作得很好。

当有人修改了别名上的remote_image时，我如何获取一个新的图像?其次，是否有更好的方法来缓存远程映像?

当前回答

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 = []

2009-09-01 09:46:03

其他回答

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

2009-08-31 14:56:16

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 = []

2009-09-01 09:46:03

如果您对重写save方法不感兴趣，可以这样做

  model_fields = [f.name for f in YourModel._meta.get_fields()]
  valid_data = {
        key: new_data[key]
        for key in model_fields
        if key in new_data.keys()
  }

  for (key, value) in valid_data.items():
        if getattr(instance, key) != value:
           print ('Data has changed')

        setattr(instance, key, value)

 instance.save()

2019-02-04 13:49:47

我有点晚了，但我也找到了这个解决方案: Django脏域

2017-01-19 07:37:01

如果你正在使用一个表单，你可以使用表单的changed_data (docs):

class AliasForm(ModelForm):

    def save(self, commit=True):
        if 'remote_image' in self.changed_data:
            # do things
            remote_image = self.cleaned_data['remote_image']
            do_things(remote_image)
        super(AliasForm, self).save(commit)

    class Meta:
        model = Alias

2016-07-15 04:02:30

保存时，如何检查字段是否已更改?

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