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ForeignKey 与 on_delete

建模一对多关联及删除行为

ForeignKey 与 on_delete 是 CoddyKit 上的免费 Django Academy 课时。这是第 1 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。

Linking Two Tables

Real data connects: a book has an author, an order has a customer. A ForeignKey is how one model points to another.

One-to-Many at a Glance

A ForeignKey models a one-to-many link. One author can have many books, but each book belongs to exactly one author.

Declaring a ForeignKey

Add a field of type ForeignKey and point it at the related model. Django stores the link as an id column behind the scenes.

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

The _id Column

In the database, an author ForeignKey becomes an author_id column holding the other row's primary key. That number is the actual link.

on_delete Is Required

Every ForeignKey needs on_delete. It tells Django what to do with this row when the row it points to is deleted.

CASCADE Deletes Children

CASCADE deletes this row too when its parent goes. Remove an author and all their books vanish with them.

author = models.ForeignKey(Author, on_delete=models.CASCADE)

PROTECT Blocks Deletion

Use PROTECT to refuse the delete while related rows exist. Django raises an error instead of removing the parent.

author = models.ForeignKey(Author, on_delete=models.PROTECT)

SET_NULL Keeps the Child

SET_NULL keeps the row but clears the link, setting the column to NULL. It needs null=True on the field to work.

author = models.ForeignKey(Author, null=True, on_delete=models.SET_NULL)

SET_DEFAULT and DO_NOTHING

SET_DEFAULT swaps in the field's default, while DO_NOTHING leaves the database to decide. Reach for these only in special cases.

Accessing the Related Object

From a book instance you read the linked author straight through the field. Django fetches that row for you on demand.

book = Book.objects.first()
print(book.author.name)

Filtering Across the Link

Span the relationship in queries with the double underscore. Reach into the related model's fields directly from filter.

Book.objects.filter(author__name="Ada")

Quick Check

You delete an author but want their books to stay, just unlinked. Which on_delete value fits?

Recap

Nice work! A ForeignKey models one-to-many links, and on_delete decides each row's fate when its parent is removed. 🔗

常见问题解答

「ForeignKey 与 on_delete」课时是免费的吗？

是的 — 「ForeignKey 与 on_delete」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。

「ForeignKey 与 on_delete」这节课中我会学到什么？

建模一对多关联及删除行为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 1 节课，共 4 节。

「ForeignKey 与 on_delete」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？

能。每节 Django Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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