如何在 django 中过滤查询集的多对多

Posted 2023-02-23

【中文标题】如何在 django 中过滤查询集的多对多

【英文标题】：How to filter the ManyToMany of a queryset in django

【发布时间】：2020-04-30 20:00:46

【问题描述】：

我有两个模型，问题及其答案，我想获取所有问题以及一个单一答案，即具有该问题的支持者人数上限。如何在 Queryset 中获取此结果（我将对结果进行分页，以便使用 queryset 来简化流程）

class Question(models.Model):
    question = models.CharField(max_length = 128)
    asker = models.ForeignKey(CustomUser, on_delete = models.CASCADE, related_name="asked_questions")
    requested = models.ManyToManyField(CustomUser, related_name="answer_requests")
    followers = models.ManyToManyField(CustomUser, related_name="followed_questions")
    topics = models.ManyToManyField(Topic, related_name="questions")
    url = models.CharField(max_length = 150)
    created_at = models.DateTimeField(auto_now_add = True, blank = True)


    def __str__(self):
        return self.question

class Answer(models.Model):
    answer = models.TextField()
    author = models.ForeignKey(CustomUser, on_delete = models.CASCADE, related_name="written_answers")
    question = models.ForeignKey(Question, on_delete = models.CASCADE, related_name="answers")
    upvoters = models.ManyToManyField(CustomUser, related_name="upvoted_answers")
    created_at = models.DateTimeField(auto_now_add = True, blank = True)

【参考方案1】：

使用subquery expression 尝试这样的操作

from django.db.models import Count, Subquery, OuterRef

answer_subquery = Answer.objects.filter(question=OuterRef('pk')).annotate(upvotes=Count('upvoters')).order_by('-upvotes')

questions = Question.objects.annotate(answer=Subquery(answer_subquery.values('answer')[:1]))

【讨论】：

超级有帮助。非常感谢！