由 post_save 信号创建的带有通用外部字段的模型实例的 AttributeError

我这里有 3 个模型需要处理：SurveyQuestion、Update和Notification。每当创建或 的实例时，我都会使用post_save信号来创建模型的实例。NotificationSurveyQuestionUpdate

该Notification模型有一个GenericForeignKey，它指向创建它的模型。在通知模型中，我尝试使用设置ForeignKey为__str__创建title它的模型实例的字段。如下所示：

class Notification(models.Model):
    source_object = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    source = GenericForeignKey("source_object", "object_id")
    #more stuff

    def __str__(self):
        return f'{self.source.title} notification'

我能够从管理面板创建 SurveyQuestion 和 Update 的实例，然后（应该）创建 Notification 的实例。但是，当我Notification在 shell 中查询实例时：

from hotline.models import Notification
notifications = Notification.objects.all()
for notification in notifications:
    print (f"Notification object: {notification}")

NoneType
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
      1 for notification in notifications:
----> 2     print (notification)
File ~/ygzsey/hotline/models.py:27, in Notification.__str__(self)
     26 def __str__(self):
---> 27     return f'{self.source.title} notification'
AttributeError: 'NoneType' object has no attribute 'title'

当我查询以下实例时SurveyQuestion：

from hotline.models import SurveyQuestion
surveys = SurveyQuestion.objects.all()
for survey in surveys:
    print (f"Model: {survey.__class__.__name__}")

Model: SurveyQuestion

当我查询实例Notification并尝试打印其字段的类名ForeignKey（我将其标记为source）时，我得到了以下信息：

for notification in notifications:
    print (f"Notification for {notification.source.__class__.__name__}")

Notification for NoneType
Notification for NoneType
Notification for NoneType

因此看起来SurveyQuestion，、Update和Notification实例都保存正常，但是存在一些问题GenericForeignKey。

我已经使用post_save创建了一个实例，但是当我尝试在管理面板中保存或的实例时会出现错误：NotificationNotification(source_object=instance, start_date=instance.start_date, end_date=instance.end_date)SurveyQuestionUpdate

ValueError at /admin/hotline/update/add/
Cannot assign "<Update: Update - ad>": "Notification.source_object" must be a "ContentType" instance.

所以我将其改为Notification(source_object=ContentType.objects.get_for_model(instance), start_date=instance.start_date, end_date=instance.end_date)。

我的完整models.py：

from django.db import models
from datetime import timedelta
from django.utils import timezone
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
from django.contrib.contenttypes.models import ContentType
from django.db.models.signals import post_save
from django.dispatch import receiver

def tmrw():
    return timezone.now() + timedelta(days=1)

class Notification(models.Model):
    source_object = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    source = GenericForeignKey("source_object", "object_id")
    start_date = models.DateTimeField(default=timezone.now)
    end_date = models.DateTimeField(default=tmrw)

    class Meta:
        verbose_name = 'Notification'
        verbose_name_plural = f'{verbose_name}s'

    def __str__(self):
        return f'{self.source.title} notification'

class Update(models.Model):
    title = models.CharField(max_length=25)
    update = models.TextField()
    start_date = models.DateTimeField(default=timezone.now)
    end_date = models.DateTimeField(default=tmrw)
    #notification = GenericRelation(Notification, related_query_name='notification')

    class Meta:
        verbose_name = 'Update'
        verbose_name_plural = f'{verbose_name}s'

    def __str__(self):
        return f'{self.__class__.__name__} - {self.title}'

class SurveyQuestion(models.Model):
    title = models.CharField(max_length=25)
    question = models.TextField()
    start_date = models.DateTimeField(default=timezone.now)
    end_date = models.DateTimeField(default=tmrw)
    #notification = GenericRelation(Notification, related_query_name='notification')

    class Meta:
        verbose_name = 'Survey'
        verbose_name_plural = f'{verbose_name}s'

    def __str__(self):
        return f'{self.__class__.__name__} - {self.title}'

class SurveyOption(models.Model):
    survey = models.ForeignKey(SurveyQuestion, on_delete=models.CASCADE, related_name='options')
    option = models.TextField()
    id = models.AutoField(primary_key=True)

    class Meta:
        verbose_name = 'Survey option'
        verbose_name_plural = f'{verbose_name}s'

    def __str__(self):
        return f'{self.survey.title} option #{self.id}'

@receiver(post_save)
def create_notification(instance, **kwargs):
    #"""
    print (f"instance: {instance}")
    print (f"instance.__class__: {instance.__class__}")
    print (f"instance.__class__.__name__: {instance.__class__.__name__}")
    #"""
    senders = ['SurveyQuestion', 'Update']
    if instance.__class__.__name__ in senders:
        notification = Notification(source_object=ContentType.objects.get_for_model(instance), start_date=instance.start_date, end_date=instance.end_date)
        notification.save()

post_save.connect(create_notification)