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定义关系与外键

使用一对多关联连接模型

定义关系与外键 是 CoddyKit 上的免费 Flask Academy 课时。这是第 3 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 Flask Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Flask 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。

Rows That Belong Together

Real data connects: a user has many posts, an order has many items. A relationship links these rows in your models. 🔗

The Foreign Key

A foreign key is a column on the child that stores the parent's id. It is the actual link saved in the database.

user_id = db.Column(db.Integer, db.ForeignKey("user.id"))

Point at the Right Table

Inside ForeignKey you name the target as table.column, using the lowercase table name Flask-SQLAlchemy generates.

db.ForeignKey("user.id")

Add a relationship

The relationship() helper turns that key into easy navigation, so you reach linked objects as attributes.

posts = db.relationship("Post")

One-to-Many in Practice

Put the foreign key on the many side and the relationship on the one side. One user, many posts.

class Post(db.Model):
    user_id = db.Column(db.ForeignKey("user.id"))

Navigate Parent to Children

Once linked, read a parent's children like a list. SQLAlchemy fetches them for you on access. ✨

user = User.query.first()
for p in user.posts:
    print(p.title)

Walk Back with backref

Add backref to a relationship and the child instantly gains a way to reach its parent.

posts = db.relationship("Post", backref="author")

Use the Backref

That backref lets you go from a child straight back to its owner with a clean attribute. 🙌

post = Post.query.first()
print(post.author.name)

Assign by Object

Link rows by setting the relationship attribute to an object. SQLAlchemy fills in the foreign key for you.

post = Post(title="Hi")
post.author = user

Cascade on Delete

Set a cascade so deleting a parent can also remove its children, keeping the database tidy and consistent.

db.relationship("Post", cascade="all, delete")

Key vs Relationship

The foreign key stores the link in SQL; relationship is the Python convenience layer on top of it. You usually want both.

Quick Check

One user can write many posts. Where does the foreign key column live?

Recap: Linking Models

You now connect tables with a foreign key, navigate them via relationship and backref, and control deletes with cascade. Models, united! 🎉

常见问题解答

「定义关系与外键」课时是免费的吗？

是的 — 「定义关系与外键」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 Flask Academy 课程的其余内容，请升级到 CoddyKit PRO。 Flask Academy 课程共包含 4 节课。

「定义关系与外键」这节课中我会学到什么？

使用一对多关联连接模型你通过在浏览器中直接运行的动手代码来练习 Flask 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 Flask Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Flask 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 3 节课，共 4 节。

「定义关系与外键」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 Flask Academy 课中编写并运行代码吗？

能。每节 Flask 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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