如何通过模型编写与自身具有多对多关系的 Django 模型

Question

我想要一个与其自身具有 ManyToMany 关系的模型，我不知道如何编写它，但我会尝试编写一些代码来说明我想要做什么。

class Person(models.Model):
   name = models.CharField()
   occupation = models.CharField()

   friends = models.ManyToManyField('self', through = PersonFriends)

我希望朋友们在 ManyToMany 字段中通过通过关系浏览我的模型

class PersonFriends(models.Model)
   ???
   comment = models.CharField()

，如果另一个模型的名称是“Pet”，例如，我会在其中命名我的字段通过类 person 和 pet 并使它们成为模型。例如，ForeignKey(Person) 和 Pet

在我的 PersonFriends 模型中为两个人命名我的字段 -现在它们是同一模型的字段？

Answer 1

你可以这样做：

class Person(models.Model):
    name = models.CharField(max_length = 255)
    occupation = models.CharField(max_length = 255)
    friends = models.ManyToManyField('self', through = 'PersonFriends', 
          symmetrical = False)
    #     ^^^^^^^^^^^
    # This has to be false when using `through` models. Or else your 
    # model will not validate.

class PersonFriends(models.Model):
    source = models.ForeignKey(Person, related_name = 'source')
    #                                  ^^^^^^^^^^^^
    # You need different `related_name` for each when you have 
    # multiple foreign keys to the same table. 

    target = models.ForeignKey(Person, related_name = 'target')
    comment = models.CharField(max_length = 255)

Answer 2

一切都在 官方文档中进行了描述ManyToManyField.through_fields （您可以在那里搜索“递归关系”短语以快速找到您需要的内容）：

对于 django 1.11，您必须指定 through 和（！） through_fields 参数：

class Person(models.Model):
    name = models.CharField(max_length=50)

    # note the additional arguments here
    friends = models.ManyToManyField(
        'self',

        # recursive relationships to self with intermediary
        # through model are always defined as non-symmetrical
        symmetrical=False,

        through='PersonFriend',

        # this argument is required to define a custom
        # through model for many to many relationship to self
        # position matters: 1 - source (from), 2 - target (to)
        through_fields=('person', 'friend'),        
    )


class PersonFriend(models.Model):
    # required relationship-defining foreign keys
    # (note that the order does not matter, it matters
    # in 'through_fields' argument in 'friends' field of the 'Person' model)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    friend = models.ForeignKey(Person, on_delete=models.CASCADE)

    # additional fields
    comment = models.CharField()

Answer 3

不假设友谊是对称的。因为巴斯光年可能是伍迪的朋友，但伍迪直到电影接近尾声才与巴斯光年成为朋友。您可以简化这两个模型，并且仍然具有合理的查找名称。如果是良好的友谊，您当然需要确保定义两个 PersonFriends。

class Person(models.Model):
   name = models.CharField()
   occupation = models.CharField()

class PersonFriends(models.Model):
    from_person = models.ForeignKey(Person, related_name='friends_with')
    to_person = models.ForeignKey(Person, related_name='friends')
    comment = models.CharField()
    class Meta:
        unique_together = ('from_person', 'to_person')

这还有一个额外的好处，那就是对友谊的每个方向都有评论。即提利昂认为珊莎是一个可爱又聪明，但却迷失的女孩。而珊莎可能会认为提利昂是一个丑陋但聪明且心地善良的人。

Answer 4

class PersonFriends(models.Model):
    from_person = models.ForeignKey(Person, related_name='from_person')
    to_person = models.ForeignKey(Person, related_name='to_person')

这是来自我的模型结构中与自身的多对多关系的数据库表结构。 Django 是这样定义的..