Django：保存后访问ManyToManyField对象

这让我感到困惑......当我保存模型时，书籍对象没有改变。但如果我打开发票并再次保存，更改就会发生。我究竟做错了什么？

class Invoice(models.Model):
    ...
    books = models.ManyToManyField(Book,blank=True,null=True)
    ...

    def save(self, *args, **kwargs):
        super(Invoice, self).save(*args, **kwargs)
        for book in self.books.all():
            book.quantity -= 1
            if book.quantity == 0:
                book.sold = True;
            book.save()

Edit:我尝试过使用 post_save 信号，但其工作方式相同。第一次保存时没有更改，第二次保存更改。

Update:似乎可以用这段代码解决：

class InvoiceAdmin(admin.ModelAdmin):
    ...

    def save_model(self, request, obj, form, change):
        obj.save()
        for bk in form.cleaned_data['books']:
            book = Book.objects.get(pk=bk.id)
            book.quantity -= 1
            if book.quantity == 0:
                book.sold = True;
            book.save()

这就是我解决这种令人困惑的行为的方法。将信号接收器连接到 models.signals.m2m_changed 事件，每次 m2m 字段更改时都会触发此事件。内嵌注释解释了原因。

class Gig(models.Model):
    def slugify(self):
        # Add venue name, date and artists to slug
        self.slug = slugify(self.venue.name) + "-"
        self.slug += self.date.strftime("%d-%m-%Y") + "-"
        self.slug += "-".join([slugify(artist.name) for artist in self.artists.all()]) 
        self.save()


@receiver(models.signals.m2m_changed, sender=Gig.artist.through)
def gig_artists_changed(sender, instance, **kwargs):
    # This callback function get's called twice. 
    # 1 first change appears to be adding an empty list
    # 2nd change is adding the actual artists
    if instance.artist.all() and not instance.slug:                                                                                                                                                               
        instance.slugify()