Database migrations with Flask-Migrate - Mini Project: Build & Apply

Database migrations with Flask-Migrate

📖 Scenario: You are building a simple Flask web app to manage a list of books. You want to keep your database structure organized and easy to update as your app grows.

🎯 Goal: Learn how to set up Flask-Migrate to handle database migrations. You will create the initial database model, configure Flask-Migrate, generate a migration script, and apply it to update the database.

📋 What You'll Learn

Create a Flask app with SQLAlchemy and a Book model

Configure Flask-Migrate with the Flask app and database

Generate an initial migration script for the Book model

Apply the migration to create the books table in the database

💡 Why This Matters

🌍 Real World

Database migrations help developers update database structures without losing data. This is essential when apps grow and change.

💼 Career

Knowing Flask-Migrate is important for backend developers working with Flask apps to maintain and evolve databases safely.

1

Create the Flask app and Book model

Create a Flask app instance called app. Set up SQLAlchemy with db = SQLAlchemy(app). Define a Book model with id as an integer primary key and title as a string column.

Flask

# Create Flask app and Book model
# Your code here

Hint

Remember to import Flask and SQLAlchemy. Set the database URI to use SQLite file books.db.

2

Configure Flask-Migrate

Import Migrate from flask_migrate. Create a migrate object with migrate = Migrate(app, db) to link Flask-Migrate with your app and database.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)

# Configure Flask-Migrate here
# Your code here

Hint

Import Migrate and create the migrate object after setting up db.

3

Generate the initial migration script

Use the Flask CLI command flask db init to create the migrations folder. Then run flask db migrate -m "Initial migration" to generate the migration script for the Book model.

Flask

# Run the following commands in your terminal:
# flask db init
# flask db migrate -m "Initial migration"

Hint

These commands are run in the terminal, not in Python code. They create migration files for your database changes.

4

Apply the migration to update the database

Run the Flask CLI command flask db upgrade in your terminal to apply the migration and create the books table in the database.

Flask

# Run the following command in your terminal:
# flask db upgrade

Hint

This command applies the migration changes to your database. Run it in the terminal.

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