Database migration in deployment in Flask - Step-by-Step Execution

Concept Flow - Database migration in deployment

Write migration script

↓

Generate migration files

↓

Review migration files

↓

Apply migration to local DB

↓

Commit migration files

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Deploy to server

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Run migration command on server

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Database schema updated

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App uses new schema

This flow shows how migration scripts are created, tested locally, committed, deployed, and applied to update the database schema safely.

Execution Sample

Flask

from flask_migrate import Migrate, upgrade

migrate = Migrate(app, db)

# On server:
upgrade()

This code initializes migration support and runs the upgrade command to apply migrations on deployment.

Execution Table

Step	Action	Command/Code	Result	Notes
1	Write migration script	flask db migrate -m 'Add users table'	Migration file created	Defines changes to DB schema
2	Review migration file	Open migration script	Confirmed changes correct	Check for errors
3	Apply migration locally	flask db upgrade	Local DB schema updated	Test migration works
4	Commit migration files	git add migrations/ && git commit -m 'Add migration files'	Files saved in repo	Ready for deployment
5	Deploy app to server	git push and deploy	New code and migrations on server	Server ready for migration
6	Run migration on server	flask db upgrade	Server DB schema updated	Apply changes safely
7	App runs with new schema	Start app	App uses updated DB	Deployment complete

💡 All migration steps completed, database schema is up to date on server

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 6	Final
Migration files	None	Created	Created	Created	Created
Local DB schema	Old	Old	Updated	Updated	Updated
Server DB schema	Old	Old	Old	Updated	Updated

Key Moments - 3 Insights

Why do we run 'flask db migrate' before 'flask db upgrade'?

Because 'flask db migrate' creates the migration script describing changes, and 'flask db upgrade' applies those changes to the database. See execution_table steps 1 and 3.

Why test migration locally before deploying?

To ensure the migration script works correctly and does not break the database. This is shown in execution_table step 3.

What happens if we forget to run 'flask db upgrade' on the server?

The server database schema stays old, causing errors when the app expects new tables or columns. See execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution table, what command creates the migration script?

A flask db upgrade

B flask db migrate -m 'Add users table'

C git commit migrations/

D Start app

💡 Hint

Check Step 1 in the execution_table where migration file is created

At which step is the local database schema updated?

A Step 6

B Step 1

C Step 3

D Step 7

💡 Hint

Look at Step 3 in execution_table where local DB schema changes

If we skip committing migration files, what happens on deployment?

A Migration files missing on server, upgrade fails

B Local DB schema updates automatically

C Server applies migrations successfully

D App runs without database

💡 Hint

See Step 4 and 5 in execution_table about committing and deploying migration files

Concept Snapshot

Database migration in Flask:
1. Use 'flask db migrate' to create migration scripts.
2. Review and test locally with 'flask db upgrade'.
3. Commit migration files to version control.
4. Deploy code and run 'flask db upgrade' on server.
5. This updates the database schema safely without data loss.

Full Transcript

Database migration in deployment with Flask involves creating migration scripts that describe changes to the database schema. First, you run 'flask db migrate' to generate these scripts. Then, you review and apply them locally using 'flask db upgrade' to test. After confirming, you commit the migration files to your code repository. When deploying, you push the code and migration files to the server. On the server, running 'flask db upgrade' applies the schema changes to the production database. This process ensures the database structure matches the app's expectations without losing data or causing errors.