如何确保 Django 中多字段的唯一性

Question

假设您有以下模型：

class User(models.Model):

class Tag(models.Model):
    text = models.CharField(unique=True)

class Subscription(models.Model):
    owner = models.ForeignKey(User)
    tags = models.ManyToManyField(Tag)
    class Meta:
        unique_together = (("owner", "tags"),)

class Post(models.Model):
    owner = models.ForeignKey(User)
    tags = models.ManyToManyField(Tag)

从上面可以看出，用户可以根据标签进行订阅。理想情况下，您想确保用户的每组订阅都是唯一的吗？这就是我将 unique_together 添加到元类的原因。但我收到了这个错误：

Subscription: (models.E013) 'unique_together' refers to a ManyToManyField 'tags', but ManyToManyFields are not permitted in 'unique_together'.

有什么想法吗？谢谢。

Answer 1

你不需要指定 unique_together。默认情况下，Django 中的 ManyToManyField 是唯一的。尝试添加相同的关系两次，您会发现只有一条记录。

有一个有用的 Django Snippet 对此我已经为您的示例进行了修改：

from django.db import models
from django.db.models.signals import m2m_changed
from django.dispatch import receiver
from django.db.utils import IntegrityError

class User(models.Model):
    pass

class Tag(models.Model):
    text = models.CharField(unique=True)

class Subscription(models.Model):
    owner = models.ForeignKey(User)
    tags = models.ManyToManyField(Tag)
    class Meta:
        unique_together = (("owner", "tags"),)

class Post(models.Model):
    owner = models.ForeignKey(User)
    tags = models.ManyToManyField(Tag)

@receiver(m2m_changed, sender=Subscription.tags.through)
def verify_uniqueness(sender, **kwargs):
    subscription = kwargs.get('instance', None)
    action = kwargs.get('action', None)
    tags = kwargs.get('pk_set', None)

    if action == 'pre_add':
        for tag in tags:
            if Subscription.objects.filter(owner=subscription.owner).filter(tag=tag):
                raise IntegrityError('Subscription with owner %s already exists for tag %s' % (subscription.owner, tag))