检查查询是否存在关系

Question

我正在尝试使用查询检查是否存在以下关系。 首先，获取用户拥有的所有关注者，然后检查用户是否关注那些关注者。 这是我的模型：

class Following(models.Model):
    target = models.ForeignKey('User', related_name='followers', on_delete=models.CASCADE, null=True)
    follower = models.ForeignKey('User', related_name='targets', on_delete=models.CASCADE, null=True)
    created_at = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return '{} is followed by {}'.format(self.target, self.follower)

class User(AbstractBaseUser):
    username    = models.CharField(max_length=15, unique=True)

    email       = models.EmailField(max_length=100, unique=True)

我正在使用Django Rest-Framework，因此我转到特定的URL以获取所需的信息。 转到URL后，将得到预期的输出。 我得到了用户拥有的所有关注者。

views.py

class GetFollowersView(ListAPIView):
    serializer_class = FollowingSerializer

    def get_queryset(self):
        requested_user = get_requested_user(self)
        return User.objects.filter(targets__target=requested_user).order_by('-targets__created_at'). \
            annotate(is_following=Count('followers__follower', filter=Q(followers__follower=requested_user), distinct=True))

def get_requested_user(self):
    filter_kwargs = {'username': self.kwargs['username']}
    return get_object_or_404(User.objects.all(), **filter_kwargs)

serializers.py

class FollowingSerializer(serializers.ModelSerializer):
    is_following = serializers.IntegerField()
    class Meta:
        model = User
        fields = ('id', 'username', 'follower_count', 'following_count', 'is_following')

但是，问题出在is_following批注中。 我想看看用户是否关注每个特定的关注者。 如果他们关注该关注者，则is_following应该为1，否则为0。我在is_following得到不正确的结果，有没有一种方法可以检查用户是否关注每个特定的关注者？

Answer 1

如果您安装了Django Debug Toolbar，并且检查了查询以查询当前的过滤器/注释，则显示的内容（针对单个用户）

SELECT "user"."id", "user"."username", "user"."email", 
COUNT(DISTINCT T4."follower_id") AS "is_following" FROM "user" 
INNER JOIN "following" ON ( "user"."id" = "following"."follower_id" ) 
LEFT OUTER JOIN "following" T4 ON ( "user"."id" = T4."target_id" ) 
WHERE "following"."target_id" = 4 GROUP BY "user"."id", "user"."username", 
"user"."email", "following"."created_at" ORDER BY "following"."created_at" 
DESC

但是，要获得所选用户关注的用户数，您确实想要这样的东西

SELECT ue."id", ue."username", ue."email", COUNT(DISTINCT fe."target_id") AS 
"is_following" FROM "user" u inner JOIN "following" fe ON ( u."id" = 
fe."follower_id" ) inner join user ue on fe.target_id = ue.id and u.id = 4
GROUP BY ue."id", ue."username", ue."email"

我认为不可能像您所做的那样在同一查询中同时合并关注者和被关注者。 您可能会找到相交处，然后从那里开始...这样的事情..

    def get_queryset(self):
        username = self.request.query_params.get('username', None)
        requested_user = models.User.objects.get(username=username)
        following_me = models.User.objects.filter(targets__target=requested_user).order_by('-targets__created_at')
        i_follow = models.User.objects.filter(followers__follower=requested_user).order_by('-followers__created_at')
        common = following_me & i_follow
        ### Set is_following for common as 1, all others as 0.
        #......
        #......
        return following_me

Answer 2

为什么不使用M2M关系？ 看起来像这样很简单：

from django.db import models


class User(models.Model):
    name = models.CharField(max_length=200)
    followers = models.ManyToManyField('User')

    @property
    def follower_count(self):
        # How many people follow me
        return len(self.followers)

    @property
    def followee_count(self):
        # How many people I follow
        return len(self.user_set.all())

并且您可以修改get_queryset()以仅找到关注者：

User.objects.filter(followers__contains=self.request.user)

这有帮助吗？