识别django post_save信号中已更改的字段

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主题和博客,但找不到解决方案.所有这些都试图使用不是我的用例的pre_save方法或表单.django docs中的https://docs.djangoproject.com/es/1.9/ref/signals/#post-save没有提到直接的方法来做到这一点.

下面的链接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,因为我曾经ipdb调试过这个并且发现该instance变量没有has_changed下面回答中提到的属性.

Django:保存时,如何检查字段是否已更改？

Answer 1

通常,覆盖保存方法比使用信号更好.

来自两勺django:"使用信号作为最后的手段."

我同意@scoopseven关于在init上缓存原始值的答案,但是如果可能则覆盖save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 2

将其设置在__init__您的模型上,以便您可以访问它.

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

现在你可以执行以下操作:

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

Answer 3

这是一个古老的问题，但是我最近遇到了这种情况，并且通过执行以下操作来完成此任务：

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)

因为它可以捕获来自应用程序和django-admin的所有更新/保存，所以它更有效。

Answer 4

在post_save方法中，您有kwargs一个字典并保存一些信息的参数。你update_fields在里面kwargs告诉你哪些领域发生了变化。此字段存储为forzenset对象。您可以检查哪些字段更改如下：

@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

但是这个解决方案存在一个问题。例如，如果您的字段值为 10，并且您再次使用 10 更新此字段，则此字段将update_fields再次出现。

Answer 5

如果要比较保存操作之前和之后的状态，则可以使用pre_save信号，该信号为您提供实例，因为它在数据库更新后应成为实例；在pre_save中，您可以读取数据库中实例的当前状态，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 6

我迟到了，但它可以对其他人有所帮助。

我们可以为此制作自定义信号。

使用自定义信号，我们可以轻松地做这些事情：

1. 帖子是否创建
2. 帖子修改与否
3. 帖子已保存，但任何字段均未更改

   class Post(models.Model):
   # some fields 

自定义信号

**用参数制作信号**

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

使用回调函数注册信号

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

从 post-admin 发送信号

我们从 Post admin 发送信号，并在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 ')  

也可以看看：

Django Signals 官方文档