django model on_delete 将 self 传递给 models.SET()

Question

例如，我有一个 Book 模型，每本书都有其作者和标签。

def get_authors_first_tag(book):
    try:
        tag = book.author.tags.first()
    except:
        return None
    else:
        return tag.id


class Author(models.Model):
    name = models.CharField(max_length=50)
    tags = models.ManyToManyField('Tag')

class Book(models.Model):
  name = models.CharField(max_length=50)
  author = models.ForeignKey(Author)
  tag =  models.ForeignKey('Tag', null=True, on_delete=models.SET(get_authors_first_tag))

我想，当删除一个标签项目时，书的标签将被设置为其作者的第一个标签。这个怎么做？谢谢。 文档页面 - https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.SET

更新。 信号没有帮助

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

class Book(models.Model):
  # temporaryly set to NULL on delete, 
  # becouse for my neds I cannot use default behavior (models.CASCADE)
  tag =  models.ForeignKey('Tag', null=True, on_delete=models.SET_NULL)

@receiver(post_delete, sender=Tag)
def delete_tag(instance, **kwargs):
    for book in instance.book_set.all():
        try:
            book.tag = book.author.tags.first()
            book.save()
        except:
            pass

更新2 最终的工作解决方案是： （脏，但有效）

@receiver(post_delete, sender=Tag)
def delete_tag(instance, **kwargs):
    books = Book.objects.all()
    for book in books:
        # after Tag item deleting, book.tag is set to None
        # so if boook's tag is null, get it's author's first tag
        if not book.tag:
            book.tag = book.author.tags.first()
            book.save()

Answer 1

SET() 不接受任何参数，因此您应该使用signal 接收器：

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

@receiver(pre_delete, sender=Tag)
def reset_tag(sender, **kwargs):
    tag = kwargs['instance']
    for book in books.filter(tag=tag):
        book.tag = book.author.tags.first()
        book.save()

将此代码放入您的models.py 或确保在启动时导入您放入接收器的文件，以便接收器实际连接。