Python Django-预删除信号不更新实例'；s外键字段

我正在尝试为我的Django应用程序的

Like

型号使用

pre_delete

信号。

Like

型号有一个

Book

外键。

Book

模型有一个

numoflikes

字段。最后，我试图从我的

pre\u delete

信号中更新这个

num\u of\u likes

字段。只是我做不到

我的代码将使问题变得非常清楚，我认为（请特别注意评论和打印声明）：

图书/模型.py：

class Book(models.Model):
    num_of_likes = models.IntegerField()

class Like(models.Model):
    user = models.ForeignKey(User)
    book = models.ForeignKey(Book)

class DeleteLikeView(APIView):

    def post(self, request, book):
        book = get_object_or_404(Book, id=book)
        print(book.num_of_likes) # Prints, say, 10
        like = Like.objects.get(user=request.user, book=book)
        like.delete() # triggers signal handler below (should update `book.num_of_likes`)
        print(book.num_of_likes) # Still prints 10, expected 9 <------ PROBLEM
        return ...

@receiver(pre_delete, sender=Like)
def delete_book_like(sender, instance, **kwargs):
    print(instance.book.num_of_likes) # Prints 10
    instance.book.num_of_likes -= 1
    instance.book.save()
    print(instance.book.num_of_likes) # Prints 9, as expected

喜欢/型号。py：

class Book(models.Model):
    num_of_likes = models.IntegerField()

class Like(models.Model):
    user = models.ForeignKey(User)
    book = models.ForeignKey(Book)

class DeleteLikeView(APIView):

    def post(self, request, book):
        book = get_object_or_404(Book, id=book)
        print(book.num_of_likes) # Prints, say, 10
        like = Like.objects.get(user=request.user, book=book)
        like.delete() # triggers signal handler below (should update `book.num_of_likes`)
        print(book.num_of_likes) # Still prints 10, expected 9 <------ PROBLEM
        return ...

@receiver(pre_delete, sender=Like)
def delete_book_like(sender, instance, **kwargs):
    print(instance.book.num_of_likes) # Prints 10
    instance.book.num_of_likes -= 1
    instance.book.save()
    print(instance.book.num_of_likes) # Prints 9, as expected

喜欢查看视图。py：

class Book(models.Model):
    num_of_likes = models.IntegerField()

class Like(models.Model):
    user = models.ForeignKey(User)
    book = models.ForeignKey(Book)

class DeleteLikeView(APIView):

    def post(self, request, book):
        book = get_object_or_404(Book, id=book)
        print(book.num_of_likes) # Prints, say, 10
        like = Like.objects.get(user=request.user, book=book)
        like.delete() # triggers signal handler below (should update `book.num_of_likes`)
        print(book.num_of_likes) # Still prints 10, expected 9 <------ PROBLEM
        return ...

@receiver(pre_delete, sender=Like)
def delete_book_like(sender, instance, **kwargs):
    print(instance.book.num_of_likes) # Prints 10
    instance.book.num_of_likes -= 1
    instance.book.save()
    print(instance.book.num_of_likes) # Prints 9, as expected

---

为什么

book.num of_likes

在

delete_book_like

中得到更新，但是更改不会显示在

DeleteLikeView

中？

删除视图中的

book

与信号处理程序中的

实例.book

不同。Python对象不会神奇地发现底层db表示已经更改。打印前，您可以从\u db调用

refresh\u

：

 book.refresh_from_db()
 print(book.num_of_likes)

或者只需将

num\u of_likes

作为一个动态计算的属性，您就不必担心非规范化数据的完整性：

class Book(models.Model):
    @property
    def num_of_likes(self):
        return self.like_set.count()

---

delete视图中的

book

与信号处理程序中的

instance.book

是不同的Python对象。Python对象不会神奇地发现底层db表示已经更改。打印前，您可以从\u db调用

refresh\u

：

 book.refresh_from_db()
 print(book.num_of_likes)

或者只需将

num\u of_likes

作为一个动态计算的属性，您就不必担心非规范化数据的完整性：

class Book(models.Model):
    @property
    def num_of_likes(self):
        return self.like_set.count()

---

谢谢你的解决方案。你能给我指一些关于你推荐的@property decorator的文档吗？谢谢，总是一个好的开始。您可能还对感谢您的解决方案感兴趣。你能给我指一些关于你推荐的@property decorator的文档吗？谢谢，总是一个好的开始。您可能也对以下内容感兴趣：