django 视图部分中的代码混淆

Question

class Item(models.Model):
    title = models.CharField(max_length=100)
    price = models.FloatField()
    discount_price = models.FloatField(blank=True, null=True)
    category = models.CharField(choices=category_choices, max_length=2)
    label = models.CharField(choices=label_choices, max_length=1)
    slug = models.SlugField()
    description = models.TextField()


    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse("product", kwargs={
            'slug': self.slug
        })

    def get_add_to_cart_url(self):
        return reverse("add-to-cart", kwargs={
            'slug': self.slug
        })



class OrderItem(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE,null=True)
    ordered = models.BooleanField(default=False)
    item = models.ForeignKey(Item, on_delete=models.CASCADE)
    quantity = models.IntegerField(default=1)

    def __str__(self):
        return f"{self.quantity} of {self.item.title}"




class Order(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE)

    items = models.ManyToManyField(OrderItem)
    start_date = models.DateTimeField(auto_now_add=True)
    ordered_date = models.DateTimeField()
    ordered = models.BooleanField(default=False)

    def __str__(self):
        return self.user.username




 def add_to_cart(request, slug):
        item = get_object_or_404(Item, slug=slug)
        order_item, created = OrderItem.objects.get_or_create(
            item=item,
            user=request.user,
            ordered=False
        )
        order_qs = Order.objects.filter(user=request.user, ordered=False)
        if order_qs.exists():
            order = order_qs[0]
            # check if the order item is in the order
            if order.items.filter(item__slug=item.slug).exists():
                order_item.quantity += 1
                order_item.save()
            else:
                order.items.add(order_item)
        else:
            ordered_date = timezone.now()
            order = Order.objects.create(user=request.user, ordered_date=ordered_date)
            order.items.add(order_item)
        return redirect("product",slug=slug)

我正在学习它。在上面我正在尝试将产品添加到购物车。首先，我将获取特定 slug 的对象，然后将其存储在 OrderItem 类中。但我很困惑：如果 order.items.filter(item__slug=item.slug).exists()：这条线有效。 slug 前的双取消划线有什么作用？

Answer 1

这是一种跨越关系的查找。 documentation 在这方面是一流的。

基本上，在这种情况下，您可以想象双下划线所在的 .。

order.items.filter(item__slug=some_slug)

会是这样的（但不要这样做，它很慢！）

[item for item in order.items if item.slug == some_slug]