Django：尝试访问 ManyToMany 字段时出错

Question

我正在尝试创建一个类似 Twitter 的应用程序作为练习项目，并遵循此 video tutorial。我在我的应用程序中创建了以下模型。

class Profile(models.Model):
    user = models.OneToOneField(to=User, on_delete=models.CASCADE)
    bio = models.CharField(max_length=160, blank=True)
    profile_photo = models.ImageField(blank=True, null=True)
    followers = models.ManyToManyField("self", through="Relationship", related_name="follow_to",
                                       symmetrical=False, blank=True)


class Relationship(models.Model):
    user_followed = models.ForeignKey("User", related_name="followed", on_delete=models.CASCADE)
    followed_by = models.ForeignKey("Profile", related_name="follower", on_delete=models.CASCADE)
    timestamp = models.DateTimeField(auto_now=True)

迁移运行成功。

在此之后进行测试，我创建了两个用户（user1 和 user2）及其各自的个人资料，并让 user2 跟随 user1。这是它的代码-

Relationship.objects.create(user_followed=user1,followed_by=user2.profile)

此关系已成功创建。我尝试使用以下代码来获取 user1-

的关注者

user1.profile.followers.all()

但是上面的代码给出了以下错误-

Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/Users/mayank/.pyenv/versions/dwitter/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 535, in __get__
return self.related_manager_cls(instance)
File "/Users/mayank/.pyenv/versions/dwitter/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 821, in __init__
self.target_field_name = rel.field.m2m_reverse_field_name()
File "/Users/mayank/.pyenv/versions/dwitter/lib/python3.8/site-packages/django/db/models/fields/related.py", line 1554, in _get_m2m_reverse_attr
return getattr(self, cache_attr)
AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'

我试过here提到的方法，但它也给出了同样的错误。我不确定我在哪里犯了错误。如果有人能指正我，我将不胜感激。

Answer 1

因为你从 ManyToManyField 到 'self' 两个 ForeignKey 应该指向 Profile:

class Relationship(models.Model):
    <b>followed</b> = models.ForeignKey(
        <b>'Profile'</b>,
        related_name='followed',
        on_delete=models.CASCADE
    )
    followed_by = models.ForeignKey(
        <b>'Profile'</b>,
        related_name='follower',
        on_delete=models.CASCADE
    )
    timestamp = models.DateTimeField(auto_now=True)

因为现在两个 ForeignKey 都指向同一个模型，所以 source 字段和 target 字段是。您可以通过使用 through_fields=… parameter [Django-doc]:

来解决这个问题

class Profile(models.Model):
    user = models.OneToOneField(to=User, on_delete=models.CASCADE)
    bio = models.CharField(max_length=160, blank=True)
    profile_photo = models.ImageField(blank=True, null=True)
    followers = models.ManyToManyField(
        'self',
        through='Relationship',
        related_name='follow_to',
        <b>related_fields=('followed', 'followed_by')</b>
        symmetrical=False,
        blank=True
    )

因此，您可以创建一个关注者 Relation：

Relationship.objects.create(
    <b>followed</b>=<b>user1.profile</b>,
    followed_by=user2.profile
)

Django：尝试访问 ManyToMany 字段时出错

Django: error while trying to access ManyToMany field

django

django-models

django-orm

manytomanyfield