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.

Practice

1. What is the main purpose of database migration in a Flask application deployment?

easy

A. To speed up the app by removing unused tables

B. To delete all existing data before deploying

C. To safely update the database structure as the app changes

D. To backup the database automatically

Solution

1. Step 1: Understand database migration purpose

   Database migration updates the database schema to match app changes without losing data.

2. Step 2: Compare options

   Only To safely update the database structure as the app changes describes safe schema updates; others describe unrelated or harmful actions.

3. Final Answer:

   To safely update the database structure as the app changes -> Option C

4. Quick Check:

   Migration = safe schema update [OK]

Hint: Migration means changing database structure safely [OK]

Common Mistakes:

• Thinking migration deletes data
• Confusing migration with backup
• Assuming migration speeds up app

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

easy

A. flask db migrate

B. flask db create

C. flask db upgrade

D. flask db init

Solution

1. Step 1: Identify command for starting migrations

   The flask db init command creates the migration folder and scripts for the first time.

2. Step 2: Check other commands

   migrate creates migration files, upgrade applies them, create is invalid.

3. Final Answer:

   flask db init -> Option D

4. Quick Check:

   Init = start migration scripts [OK]

Hint: Init means start or create migration setup [OK]

Common Mistakes:

• Using migrate before init
• Confusing upgrade with init
• Typing create instead of init

3. Given these commands run in order:
flask db init
flask db migrate -m "Add user table"
flask db upgrade
What happens after the last command?

medium

A. The database schema updates with the new user table

B. The migration folder is deleted

C. The app restarts automatically

D. The migration message is ignored

Solution

1. Step 1: Understand each command effect

   init sets up migration scripts, migrate creates migration files, upgrade applies changes to the database.

2. Step 2: Identify final effect

   After upgrade, the database schema updates to include new tables or changes.

3. Final Answer:

   The database schema updates with the new user table -> Option A

4. Quick Check:

   Upgrade = apply schema changes [OK]

Hint: Upgrade applies migration changes to database [OK]

Common Mistakes:

• Thinking upgrade deletes migration files
• Assuming app restarts automatically
• Ignoring migration message

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

medium

A. You forgot to run flask db init first

B. Your database is already up to date with no changes

C. You ran flask db upgrade instead

D. You need to restart the Flask app

Solution

1. Step 1: Understand migrate command behavior

   flask db migrate creates migration files only if there are schema changes detected.

2. Step 2: Analyze no new file case

   If no migration file appears, it means no schema changes were found, so no new migration is needed.

3. Final Answer:

   Your database is already up to date with no changes -> Option B

4. Quick Check:

   No changes = no migration file [OK]

Hint: No migration file means no schema changes detected [OK]

Common Mistakes:

• Skipping init causes errors but not no file
• Confusing upgrade with migrate
• Thinking app restart affects migration files

5. You deployed a Flask app and added a new column to a model. Which sequence of commands correctly updates the production database without losing data?

hard

A. flask db migrate -m "Add column" -> flask db upgrade

B. flask db upgrade -> flask db migrate -m "Add column"

C. flask db init -> flask db upgrade

D. flask db migrate -m "Add column" -> flask db init

Solution

1. Step 1: Understand correct migration order

   First, create migration files with flask db migrate describing changes, then apply them with flask db upgrade.

2. Step 2: Check options for correct sequence

   flask db migrate -m "Add column" -> flask db upgrade follows this order; others reverse or misuse commands.

3. Final Answer:

   flask db migrate -m "Add column" -> flask db upgrade -> Option A

4. Quick Check:

   Migrate then upgrade = safe update [OK]

Hint: Always migrate before upgrade to apply changes safely [OK]

Common Mistakes:

• Running upgrade before migrate
• Running init after migrate
• Skipping migrate step