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ManyToManyField 与 Through 模型

通过额外数据双向关联记录

ManyToManyField 与 Through 模型 是 CoddyKit 上的免费 Django Academy 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Django Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Django Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Both Sides Have Many

Sometimes each side links to many of the other: a post has many tags, a tag marks many posts. That is a many-to-many relationship.

Declaring ManyToManyField

Add a ManyToManyField pointing at the other model. Django handles the link from both directions automatically.

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

The Hidden Join Table

Behind the scenes Django builds a join table holding pairs of ids. No new column lands on either of your two models.

Adding Relations

Once a post is saved, attach tags with add. You can pass several objects at once in a single call.

post.tags.add(python_tag, web_tag)

Removing and Clearing

Drop a single link with remove, or wipe them all with clear. The related objects themselves stay safe.

post.tags.remove(web_tag)
post.tags.clear()

Reading the Set

Access linked rows through the field like a normal manager. Call all to loop over everything connected.

for tag in post.tags.all():
    print(tag.name)

When the Link Needs Data

What if a membership has its own facts, like a join date? A plain join table cannot hold extra columns, so you need more control.

The through Model

A through model is your own table for the relationship. It lets each link carry extra fields you define.

members = models.ManyToManyField(User, through="Membership")

Defining the through Class

The through model holds a ForeignKey to each side plus any extra fields. Those columns describe the connection itself.

class Membership(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    joined = models.DateField()

add Stops Working

With a custom through model, plain add is disabled. You create the link by making a through row directly instead.

Membership.objects.create(user=u, group=g, joined=today)

Querying Across M2M

Filter through a many-to-many link with the double underscore, just like a ForeignKey. The join table is handled for you.

Post.objects.filter(tags__name="python")

Quick Check

Your group membership needs to record when each user joined. What do you reach for?

Recap

Well done! A ManyToManyField links both sides freely, and a through model lets each connection carry its own data. 🏷️

常见问题解答

「ManyToManyField 与 Through 模型」课时是免费的吗？

是的 — 「ManyToManyField 与 Through 模型」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Django Academy 课程的其余内容，请升级到 CoddyKit PRO。 Django Academy 课程共包含 4 节课。

「ManyToManyField 与 Through 模型」这节课中我会学到什么？

通过额外数据双向关联记录你通过在浏览器中直接运行的动手代码来练习 Django Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Django Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Django Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「ManyToManyField 与 Through 模型」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Django Academy 课中编写并运行代码吗？

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