Database migration in deployment in Flask

Introduction

Database migration helps you update your app's database structure safely when you change your code. It keeps your data and app working well together.

When you add a new feature that needs new tables or columns in the database.

When you fix or change how data is stored without losing existing data.

When you move your app to a new server or environment and need the database set up.

When you want to keep track of database changes over time in a team project.

Syntax

Flask

flask db init
flask db migrate -m "message"
flask db upgrade

flask db init creates migration setup files once per project.

flask db migrate creates a migration script based on model changes.

Examples

Run this once to set up migration files in your Flask project.

Flask

flask db init

This creates a migration script describing the change to add an email column.

Flask

flask db migrate -m "Add user email column"

This applies the migration to update the database schema.

Flask

flask db upgrade

Sample Program

This Flask app uses Flask-Migrate to handle database changes. Initially, the User model has only a username. Later, we add an email column. We create migration scripts and apply them to update the database without losing data.

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 adding this new column, run:
# flask db migrate -m "Add email to User"
# flask db upgrade

# Updated User model with new email column
class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=True)

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

Important Notes

Always back up your database before running migrations in production.

Use descriptive messages in flask db migrate -m to track changes clearly.

Test migrations in a development environment before applying them live.

Summary

Database migration updates your database structure safely as your app changes.

Use Flask-Migrate commands: init, migrate, and upgrade.

This keeps your data safe and your app working smoothly during updates.