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为什么需要迁移而不是 create_all

了解手动编辑线上模式的风险

第 1 / 4 课13 个步骤

为什么需要迁移而不是 create_all 是 CoddyKit 上的免费 Flask 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 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。

Schemas Always Change

Real apps evolve, so your tables must change too. You will add columns, rename fields, and drop old ones as features grow. 🔄

create_all Only Builds Once

Remember the limit you met earlier: create_all only makes missing tables. It never touches a table that already exists.

db.create_all()

The Tempting Bad Fix

One tempting fix is drop then recreate. It works in a demo, but it deletes every row, which is unthinkable in production.

db.drop_all()
db.create_all()

Editing by Hand Is Risky

Running raw ALTER TABLE by hand on a live database is fragile. One typo or missed step can corrupt data with no easy undo.

What a Migration Is

A migration is a small script that moves your schema from one version to the next in safe, repeatable steps.

Versioned and Ordered

Each migration has a version id and points to the one before it. Together they form an ordered history of every schema change.

Forward and Backward

A migration defines two paths: upgrade applies the change, and downgrade reverses it if something goes wrong.

def upgrade():
    ...

def downgrade():
    ...

Flask-Migrate Wraps Alembic

Flask-Migrate connects your app to Alembic, the tool that detects schema changes and writes the migration scripts for you.

It Tracks the Current Version

Migrations record the current schema version inside a table called alembic_version, so the tool always knows where you are.

Safe Across Every Environment

The same migration scripts run on your laptop, in CI, and in production, so every environment ends up with an identical schema.

When to Switch

Use create_all only for the very first prototype. The moment your model starts changing, move to migrations for good.

Quick Check

Your table exists and you just added a column. Why not rely on create_all?

在交互式课程中回答

Recap

You saw why create_all cannot evolve a schema and how migrations bring safe, versioned, reversible changes. 🎉

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常见问题解答

「为什么需要迁移而不是 create_all」课时是免费的吗？

是的 — 「为什么需要迁移而不是 create_all」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。

「为什么需要迁移而不是 create_all」这节课中我会学到什么？

了解手动编辑线上模式的风险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 1 节课，共 4 节。

「为什么需要迁移而不是 create_all」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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