如何使用Django模型继承信号？

Question

我在Django中有一些模型继承级别:

class WorkAttachment(models.Model):
    """ Abstract class that holds all fields that are required in each attachment """
    work            = models.ForeignKey(Work)
    added           = models.DateTimeField(default=datetime.datetime.now)
    views           = models.IntegerField(default=0)

    class Meta:
        abstract = True


class WorkAttachmentFileBased(WorkAttachment):
    """ Another base class, but for file based attachments """
    description     = models.CharField(max_length=500, blank=True)
    size            = models.IntegerField(verbose_name=_('size in bytes'))

    class Meta:
        abstract = True


class WorkAttachmentPicture(WorkAttachmentFileBased):
    """ Picture attached to work """
    image           = models.ImageField(upload_to='works/images', width_field='width', height_field='height')
    width           = models.IntegerField()
    height          = models.IntegerField()

有许多不同的模型继承自WorkAttachmentFileBased和WorkAttachment.我想创建一个信号,它会attachment_count在创建附件时更新父作业的字段.认为为父发送者(WorkAttachment)发出的信号也会为所有继承的模型运行是合乎逻辑的,但事实并非如此.这是我的代码:

@receiver(post_save, sender=WorkAttachment, dispatch_uid="att_post_save")
def update_attachment_count_on_save(sender, instance, **kwargs):
    """ Update file count for work when attachment was saved."""
    instance.work.attachment_count += 1
    instance.work.save()

有没有办法让这个信号适用于所有继承的模型WorkAttachment？

Python 2.7,Django 1.4 pre-alpha

PS我已经尝试过我在网上找到的解决方案之一,但它对我不起作用.

Answer 1

您可以注册连接处理程序而无需sender指定.并过滤其中所需的模型.

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


@receiver(post_save)
def my_handler(sender, **kwargs):
    # Returns false if 'sender' is NOT a subclass of AbstractModel
    if not issubclass(sender, AbstractModel):
       return
    ...

参考:https://groups.google.com/d/msg/django-users/E_u9pHIkiI0/YgzA1p8XaSMJ

Answer 2

最简单的解决方案是不限制sender,而是检查信号处理程序相应的实例是否是子类:

@receiver(post_save)
def update_attachment_count_on_save(sender, instance, **kwargs):
    if isinstance(instance, WorkAttachment):
        ...

然而,这可能会产生显著的性能开销,因为每次时间任何模型保存,上述函数被调用.

我想我已经找到了最常用的Django方式:Django的最新版本建议将信号处理程序连接到一个名为的文件中signals.py.这是必要的接线代码:

your_app/__ init__.py:

default_app_config = 'your_app.apps.YourAppConfig'

your_app/apps.py:

import django.apps

class YourAppConfig(django.apps.AppConfig):
    name = 'your_app'
    def ready(self):
        import your_app.signals

your_app/signals.py:

def get_subclasses(cls):
    result = [cls]
    classes_to_inspect = [cls]
    while classes_to_inspect:
        class_to_inspect = classes_to_inspect.pop()
        for subclass in class_to_inspect.__subclasses__():
            if subclass not in result:
                result.append(subclass)
                classes_to_inspect.append(subclass)
    return result

def update_attachment_count_on_save(sender, instance, **kwargs):
    instance.work.attachment_count += 1
    instance.work.save()

for subclass in get_subclasses(WorkAttachment):
    post_save.connect(update_attachment_count_on_save, subclass)

我认为这适用于所有子类,因为它们都会YourAppConfig.ready被调用时加载(因此signals被导入).

Answer 3

你可以尝试类似的东西:

model_classes = [WorkAttachment, WorkAttachmentFileBased, WorkAttachmentPicture, ...]

def update_attachment_count_on_save(sender, instance, **kwargs):
    instance.work.attachment_count += 1
    instance.work.save()

for model_class in model_classes:
    post_save.connect(update_attachment_count_on_save, 
                      sender=model_class, 
                      dispatch_uid="att_post_save_"+model_class.__name__)

(免责声明:我没有测试过以上内容)

Answer 4

我只是使用 python 的（相对）新__init_subclass__方法做到了这一点：

from django.db import models

def perform_on_save(*args, **kw):
    print("Doing something important after saving.")

class ParentClass(models.Model):
    class Meta:
        abstract = True

    @classmethod
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        models.signals.post_save.connect(perform_on_save, sender=cls)

class MySubclass(ParentClass):
    pass  # signal automatically gets connected.

这需要 django 2.1 和 python 3.6 或更高版本。请注意，在@classmethod使用 django 模型和关联的元类时似乎需要该行，即使根据官方 python 文档不需要它。

Answer 5

post_save.connect(my_handler, ParentClass)
# connect all subclasses of base content item too
for subclass in ParentClass.__subclasses__():
    post_save.connect(my_handler, subclass)

祝你今天愉快!

Answer 6

Michael Herrmann 的解决方案绝对是最 Django 的方式。是的，它适用于所有子类，因为它们在 ready() 调用中加载。

我想为文档参考做出贡献：

在实践中，信号处理程序通常定义在与它们相关的应用程序的信号子模块中。信号接收器在应用程序配置类的 ready() 方法中连接。如果您使用receiver() 装饰器，只需在ready() 中导入信号子模块。

https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions

并添加警告：

ready() 方法在测试期间可能会执行多次，因此您可能希望防止信号重复，特别是如果您计划在测试中发送它们。

https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions

因此，您可能希望在连接函数上使用 dispatch_uid 参数来防止重复信号。

post_save.connect(my_callback, dispatch_uid="my_unique_identifier")

在这种情况下，我会做：

for subclass in get_subclasses(WorkAttachment):
    post_save.connect(update_attachment_count_on_save, subclass, dispatch_uid=subclass.__name__)

https://docs.djangoproject.com/en/dev/topics/signals/#preventing-duplicate-signals