Database migrations with Flask-Migrate

Introduction

Database migrations help you change your database structure safely as your app grows. Flask-Migrate makes this easy by tracking and applying these changes step-by-step.

When you add a new column to a database table in your Flask app.

When you rename or remove a database table or column.

When you want to share database changes with your team.

When deploying updates to your app's database on a server.

When you want to keep your database schema versioned and organized.

Syntax

Flask

from flask_migrate import Migrate

migrate = Migrate(app, db)

# Then use command line:
# flask db init
# flask db migrate -m "message"
# flask db upgrade

Flask-Migrate works with Flask and SQLAlchemy to manage database changes.

Use the command line commands to create and apply migrations.

Examples

Initializes the migrations folder to start tracking database changes.

Flask

flask db init

Creates a new migration script describing the changes you made to your models.

Flask

flask db migrate -m "Add user email column"

Applies the migration changes to your actual database.

Flask

flask db upgrade

Sample Program

This Flask app sets up a User model and uses Flask-Migrate to handle database changes. You run commands to create migration files and update the database.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///app.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

db = SQLAlchemy(app)
migrate = Migrate(app, db)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True, nullable=False)

# After saving this, run in terminal:
# flask db init
# flask db migrate -m "Create user table"
# flask db upgrade

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always review migration scripts before applying them to avoid unwanted changes.

Keep your migration files in version control to share with your team.

Use descriptive messages with flask db migrate -m "message" to remember what each migration does.

Summary

Flask-Migrate helps manage database changes safely and easily.

Use command line commands to create and apply migrations.

Keep your database schema organized as your app grows.

Practice

1. What is the main purpose of Flask-Migrate in a Flask application?

easy

A. To handle user authentication and sessions

B. To create HTML templates for the Flask app

C. To manage database schema changes safely using migrations

D. To serve static files like CSS and JavaScript

Solution

1. Step 1: Understand Flask-Migrate's role

   Flask-Migrate is a tool that helps manage changes to the database structure over time.

2. Step 2: Identify the correct purpose

   It uses migrations to safely update the database schema without losing data.

3. Final Answer:

   To manage database schema changes safely using migrations -> Option C

4. Quick Check:

   Flask-Migrate = database migrations [OK]

Hint: Flask-Migrate is about database schema changes [OK]

Common Mistakes:

• Confusing Flask-Migrate with template or static file handling
• Thinking it manages user sessions
• Assuming it creates HTML or CSS files

2. Which command correctly initializes Flask-Migrate in your Flask project?

easy

A. flask db init

B. flask migrate start

C. flask db create

D. flask migrate init

Solution

1. Step 1: Recall the initialization command

   The command to set up Flask-Migrate for the first time is flask db init.

2. Step 2: Check other options

   Other commands like flask migrate start or flask db create are incorrect or do not exist.

3. Final Answer:

   flask db init -> Option A

4. Quick Check:

   Initialize Flask-Migrate = flask db init [OK]

Hint: Use 'flask db init' to start migrations [OK]

Common Mistakes:

• Using 'flask migrate init' instead of 'flask db init'
• Confusing 'start' with 'init'
• Trying commands that don't exist in Flask-Migrate

3. Given the following commands run in order:

flask db init
flask db migrate -m "Add users table"
flask db upgrade

What is the effect on the database?

medium

A. No changes happen until the app restarts

B. The database is reset and all data is lost

C. Only a migration script is created but not applied

D. The database schema is updated with the new users table

Solution

1. Step 1: Understand each command's role

   flask db init sets up migration files, flask db migrate creates a migration script, and flask db upgrade applies changes to the database.

2. Step 2: Determine the final effect

   After running all three, the database schema is updated with the new users table as described in the migration.

3. Final Answer:

   The database schema is updated with the new users table -> Option D

4. Quick Check:

   Upgrade applies migration = schema updated [OK]

Hint: Upgrade applies migration changes to DB [OK]

Common Mistakes:

• Thinking upgrade resets the database
• Believing migrate alone applies changes
• Assuming changes need app restart

4. You run flask db migrate but see no new migration script created. What is the most likely cause?

medium

A. You forgot to run flask db init first

B. No changes detected in the models since last migration

C. The database server is down

D. You need to restart the Flask app

Solution

1. Step 1: Understand what triggers migration scripts

   flask db migrate creates a script only if it detects changes in the database models.

2. Step 2: Identify the cause of no new script

   If no changes are found, no new migration script is generated, which is the most common reason.

3. Final Answer:

   No changes detected in the models since last migration -> Option B

4. Quick Check:

   No model changes = no migration script [OK]

Hint: No model changes means no migration script [OK]

Common Mistakes:

• Assuming database server status affects migration script creation
• Thinking app restart is needed for migrations
• Forgetting to initialize migrations (less common if already done)

5. You want to rename a column in your database using Flask-Migrate. Which sequence of steps is correct?

hard

A. Update the model's column name, run flask db migrate, then flask db upgrade

B. Run flask db upgrade first, then update the model, then flask db migrate

C. Delete the old column manually, update the model, then run flask db migrate

D. Update the model, restart Flask app, then run flask db upgrade

Solution

1. Step 1: Change the model to reflect the new column name

   Modify your SQLAlchemy model to rename the column as desired.

2. Step 2: Create and apply migration

   Run flask db migrate to generate the migration script, then flask db upgrade to apply it to the database.

3. Final Answer:

   Update the model's column name, run flask db migrate, then flask db upgrade -> Option A

4. Quick Check:

   Model change + migrate + upgrade = rename column [OK]

Hint: Change model first, then migrate and upgrade [OK]

Common Mistakes:

• Running upgrade before migrate
• Manually deleting columns instead of migrating
• Restarting app without migration commands