为什么要用“通过” Django 模型中 ManyToManyField 的参数？

发布于 2024-12-26 05:35:15 字数 673 浏览 1 评论 0原文

查看 Django 文档并尝试找出“through”参数的用法。这是

示例：

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __unicode__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

为什么需要 Group 的“members”属性？难道 Membership 的“组”ForeignKey 不足以跟踪关系并访问该信息吗？

原文

Looking through the Django docs and trying to figure out the use of the "through" argument. Here is a link to the doc.

The example:

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __unicode__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

Why is the "members" attribute of Group even needed? Isn't the 'group' ForeignKey of Membership enough to follow the relation and access that information?

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薄情伤 2025-01-02 05:35:15

我认为你对这个问题的思考有点过于字面意思了。假设您没有使用through：

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person)

    def __unicode__(self):
        return self.name

Django 在幕后本质上为您创建了以下模型：

class GroupPerson(models.Model)
    group = models.ForeignKey(Group)
    person = models.ForeignKey(Person)

创建Membership 的原因model 的作用是添加 Django 自动创建的默认模型默认情况下不会有的额外数据，但由于您不再使用默认模型，因此必须告诉 Django，使用 through。基本上，您保留了 Django 为 ManyToManyFields 提供的 API。

I think you're thinking a little too literally about this. Let's say you didn't use through:

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person)

    def __unicode__(self):
        return self.name

Django, behind the scenes, essentially creates the following model for you:

class GroupPerson(models.Model)
    group = models.ForeignKey(Group)
    person = models.ForeignKey(Person)

The reason for creating a Membership model is to add extra data that the default model Django automatically creates wouldn't have by default, but since you're no longer using the default, you have to tell Django that, using through. Basically, you're preserving the API Django provides for ManyToManyFields.

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