Django 查询 - 在带注释的计数过滤器中获取父对象

Question

我有 3 个模型：

class User(AbstractBaseUser, PermissionsMixin):
    username = models.CharField(db_index=True, max_length=20, unique = True)

class Content(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, null=True) #User.content_set.all() returns all Content objects of the content
    contentURL = models.CharField(max_length=256, null=True)
    thumbnailURL = models.CharField(max_length=256, null=True, blank=True)
    second_content = models.OneToOneField('self', on_delete=models.SET_NULL, null=True, blank=True) #if this is not NULL, then the content has been uploaded with a second one and they form a pair to be retrieved together
    timestamp = models.DateTimeField(auto_now_add=True)

class Fight(models.Model):
    win_content = models.ForeignKey(Content, db_index=True, on_delete=models.SET_NULL, related_name="wins", null=True) #Content.wins.all() returns all Fight objects of the content in which this content has won
    loss_content = models.ForeignKey(Content, db_index=True, on_delete=models.SET_NULL, related_name="losses", null=True) #Content.losses.all() returns all Fight objects of the content in which this content has lost
    user = models.ForeignKey(User, on_delete=models.SET_NULL, blank=True, null=True)
    is_private = models.BooleanField(db_index=True, default=False) #we will filter those out for user quality calculations
    timestamp = models.DateTimeField(auto_now_add=True)

我想得到：

1. second_content 不为空的所有Content

2. 用相关的Fight计数注释每个内容：一次是胜利，一次是失败。

这是我的查询集：

contents = user.content_set.exclude(content__second_content=None).annotate(
    win_count=Count('wins', filter=Q(wins__loss_content=second_content)),
    loss_count=Count('losses', filter=Q(losses__win_content=second_content))
).order_by('-timestamp')

问题在于Q(wins__loss_content=second_content)。 second_content 未定义，因为它引用 Fight 对象，而不是父对象。 如何引用父对象？我试过Q(wins__loss_content=content__second_content)，还是不行！

Answer 1

您可以使用F object [Django-doc] 来引用一个字段，例如F('content') 因此引用外键字段。

因此，您可以在如下表达式中使用它：

contents = user.content_set.exclude(content__second_content=None).annotate(
    win_count=Count('wins', filter=Q(wins__loss_content=F('content'))),
    loss_count=Count('losses', filter=Q(losses__win_content=F('content')))
).order_by('-timestamp')

请注意，这里您计算两个单独的JOINs，所以也许您应该在您的Count(..) expression [Django-doc] 中添加一个distinct=True parameter [Django-doc] 以避免将相同的wins 和losses 计算两次。