Django ManyToMany通过集合大小或成员进行过滤 [英] Django ManyToMany filtering by set size or member in the set

问题描述

我使用 django 来维护消息的数据库。

其中我有以下模型：

I am using django to maintain a database of messages.
Among others I have the following models:

class User(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=10)

class Message(models.Model):
    id = models.IntegerField(primary_key=True)
    body = models.CharField(max_length=200)
    users = models.ManyToManyField(User)

我正在尝试写一个对于给定的用户给出了他（和他一个人）相关联的消息的实用方法。

I am trying to write a utility method that for a given user gives me the messages he (and he alone) is associated with.

ie for：

m1 = Message(id=1, body='Some body')
m1.save()
m2 = Message(id=2, body='Another body')
m2.save()
m3 = Message(id=3, body='And yet another body')
m3.save()

u1 = User(name='Jesse James')
u1.save()
u2 = User(name='John Doe')
u2.save()

m1.users.add(u1, u2)
m2.users.add(u1)
m3.users.add(u2)

getMessagesFor('Jesse James')

只返回 m2 。

假设我有用户正确的模型实例，它归结为一行，我已经尝试了这些：

Will return only m2.
Assuming I have in user the right model instance, it boils down to one line, and I have tried these following:

    user.message_set.annotate(usr_cnt=Count('users')).filter(usr_cnt__lte=1)

或：

    messages = Message.objects.filter(users__id__in=[user.id])

和：

    messages = Message.objects.filter(users__id__exact=user.id)

And：

    messages = Message.objects.filter(users__contains=user)

等等...我总是得到 m2 AND m1

And so on... I always get both m2 AND m1.
Tried annotations, excludes, filters etc.

有人可以帮助我吗？

推荐答案

qs = Message.objects.annotate(cc=Count('users')).filter(cc=1)

以上查询将返回只有一个用户关联的所有邮件。

Above query will return all messages which has only single user associated with it.

要按用户过滤，请根据用户添加另一个筛选器：根据用户过滤注释查询：

To filter by user, add another filter at end to filter the annotated query according to user:

qs = Message.objects.annotate(cc=Count('users')).filter(cc=1).filter(users__id=user.id)
# if user user.id=1, this will return only m2

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