识别 django post_save 信号中更改的字段

Question

我正在使用 django 的 post_save 信号在保存模型后执行一些语句。

class Mode(models.Model):
    name = models.CharField(max_length=5)
    mode = models.BooleanField()


from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save, sender=Mode)
def post_save(sender, instance, created, **kwargs):
        # do some stuff
        pass

现在我想根据mode字段的值是否发生变化来执行一条语句。

@receiver(post_save, sender=Mode)
def post_save(sender, instance, created, **kwargs):
        # if value of `mode` has changed:
        #  then do this
        # else:
        #  do that
        pass

我查看了一些 SOF 线程和博客，但找不到解决方案。他们都试图使用 pre_save 方法或表单，这不是我的用例。 https://docs.djangoproject.com/es/1.9/ref/signals/#post-save 在 django 文档中没有提到执行此操作的直接方法。

下面 link 中的答案看起来很有希望，但我不知道如何使用它。我不确定最新的 django 版本是否支持它，因为我使用 ipdb 调试它并发现 instance 变量没有属性 has_changed 如下面的答案所述.

Django: When saving, how can you check if a field has changed?

Answer 1

在您模型的 __init__ 上进行设置，以便您可以访问它。

def __init__(self, *args, **kwargs):
    super(YourModel, self).__init__(*args, **kwargs)
    self.__original_mode = self.mode

现在您可以执行如下操作：

if instance.mode != instance.__original_mode:
    # do something useful

Answer 2

通常覆盖保存方法比使用信号更好。

From Two scoops of django: "Use signals as a last resort."

我同意@scoopseven 关于在 init 上缓存原始值的回答，但如果可能的话覆盖保存方法。

class Mode(models.Model):
    name = models.CharField(max_length=5)
    mode = models.BooleanField()
    __original_mode = None

    def __init__(self, *args, **kwargs):
        super(Mode, self).__init__(*args, **kwargs)
        self.__original_mode = self.mode

    def save(self, force_insert=False, force_update=False, *args, **kwargs):
        if self.mode != self.__original_mode:
            #  then do this
        else:
            #  do that

        super(Mode, self).save(force_insert, force_update, *args, **kwargs)
        self.__original_mode = self.mode

Answer 3

如果你想比较保存操作前后的状态，你可以使用 pre_save 信号，它为你提供实例，因为它在数据库更新后应该变成这样，在 pre_save 中你可以读取当前状态数据库中的实例并根据差异执行一些操作。

from django.db.models.signals import pre_save
from django.dispatch import receiver


@receiver(pre_save, sender=MyModel)
def on_change(sender, instance: MyModel, **kwargs):
    if instance.id is None: # new object will be created
        pass # write your code here
    else:
        previous = MyModel.objects.get(id=instance.id)
        if previous.field_a != instance.field_a: # field will be updated
            pass  # write your code here

Answer 4

这是一个老问题，但我最近遇到过这种情况，我通过执行以下操作完成了它：

    class Mode(models.Model):
    
        def save(self, *args, **kwargs):
            if self.pk:
                # If self.pk is not None then it's an update.
                cls = self.__class__
                old = cls.objects.get(pk=self.pk)
                # This will get the current model state since super().save() isn't called yet.
                new = self  # This gets the newly instantiated Mode object with the new values.
                changed_fields = []
                for field in cls._meta.get_fields():
                    field_name = field.name
                    try:
                        if getattr(old, field_name) != getattr(new, field_name):
                            changed_fields.append(field_name)
                    except Exception as ex:  # Catch field does not exist exception
                        pass
                kwargs['update_fields'] = changed_fields
            super().save(*args, **kwargs)

这更有效，因为它捕获了来自应用程序和 django-admin 的所有 updates/saves。

Answer 5

在 post_save 方法中你有 kwargs 参数，它是一个字典并包含一些信息。 kwargs 中的 update_fields 告诉您更改了哪些字段。此字段存储为 forzenset 对象。您可以检查哪些字段发生了这样的变化：

@receiver(post_save, sender=Mode)
def post_save(sender, instance, created, **kwargs):
    if not created:
        for item in iter(kwargs.get('update_fields')):
            if item == 'field_name' and instance.field_name == "some_value":
               # do something here

但是这个解决方案有一个问题。例如，如果您的字段值为 10，并且您再次将此字段更新为 10，则此字段将再次处于 update_fields。

Answer 6

我来晚了，但这对其他人有帮助。

我们可以为此定制信号。

使用自定义信号我们可以轻松地做这些事情：

Post是否创建
Post是否修改
Post 已保存但任何字段未更改

   class Post(models.Model):
   # some fields

自定义信号

**用参数发出信号**

from django.dispatch import Signal, receiver
# provide arguments for your call back function
post_signal = Signal(providing_args=['sender','instance','change','updatedfields'])

注册带回调函数的信号

# register your signal with receiver decorator 
@receiver(post_signal)
def post_signalReciever(sender,**kwargs):
    print(kwargs['updatedfields'])
    print(kwargs['change'])

正在从 post-admin

发送信号

我们从 Post admin 发送信号，并在对象实际修改时保存对象

#sending the signals 
class PostAdmin(admin.ModelAdmin):
   # filters or fields goes here 

   #save method 
   def save_model(self, request, obj, form, change):


    if not change and form.has_changed():  # new  post created
        super(PostAdmin, self).save_model(request, obj, form, change)
        post_signal.send(self.__class__,instance=obj,change=change,updatedfields=form.changed_data)
        print('Post created')
    elif change and form.has_changed(): # post is actually modified )
        super(PostAdmin, self).save_model(request, obj, form, change)
        post_signal.send(self.__class__,instance=obj,change=change,updatedfields=form.changed_data)
        print('Post modified')
    elif change and not form.has_changed() :
        print('Post not created or not updated only saved ')

另请参阅：

Django Signals official doc

Answer 7

您可以在 Django 信号中使用 update_fields。

@receiver(post_save, sender=Mode)
def post_save(sender, instance, created, **kwargs):

    # only update instance
    if not created:

        update_fields = kwargs.get('update_fields') or set()

        # value of `mode` has changed:
        if 'mode' in update_fields:
            # then do this
            pass
        else:
            # do that
            pass

Answer 8

这可以用instance._state.adding

来识别

if not instance._state.adding:
    # update to existing record
    do smthng

else:
    # new object insert operation
    do smthng

识别 django post_save 信号中更改的字段

Identify the changed fields in django post_save signal

python

django

django-models

django-signals