Django：保存后访问ManyToManyField对象 [英] Django: accessing ManyToManyField objects after the save

问题描述

这让我感到困惑...当我保存我的模型时，书对象没有改变。但如果我打开发票并再次保存，则进行更改。我做错了什么？

This is baffling me... When I save my model, the book objects are unchanged. But if I open the invoice and save it again, the changes are made. What am I doing wrong?

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()

编辑：尝试使用post_save信号，但它的工作方式相同。第一次保存没有更改，第二次保存更改。

I've tried using the post_save signal, but it works the same way. No changes on the first save, changes saved the second time.

更新：似乎需要使用以下代码解决：

Update: Seems to be solved with this code:

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()

推荐答案

我如何解决这个，确实令人困惑的行为。将信号接收器连接到models.signals.m2m_changed事件，每次更改m2m字段时触发此获取。内部评论解释为什么。$ /

This is how I worked around this, indeed baffling, behavior. Connect a signal receiver to models.signals.m2m_changed event, this get's triggered each time a m2m field is changed. The inline comments explain why.

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()

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