Database migrations are essential for managing changes to your database schema over time. Flask-Migrate, which is built on top of Alembic, provides a convenient way to handle these migrations in Flask applications.

1. Database Migration: The process of changing the database schema over time, including creating, altering, and dropping tables and columns.
2. Flask-Migrate: An extension that integrates Alembic with Flask and Flask-SQLAlchemy to manage database migrations.
3. Alembic: A lightweight database migration tool for use with SQLAlchemy.

First, you need to install Flask-Migrate using pip:

Next, you need to initialize Flask-Migrate in your Flask application. Update your app.py or main application file as follows:

Run the following command to create the migrations directory:

This command sets up the necessary directory structure for Alembic.

Whenever you make changes to your models, you need to create a new migration script. For example, if you add a new model or modify an existing one, run:

This command generates a new migration script in the migrations/versions directory.

To apply the migration to your database, run:

This command applies the changes defined in the migration script to your database.

Let's add a new model called Post to our application:

After adding the model, create and apply the migration:

1. Task: Add a new column updated_at to the Post model to track the last update time.
2. Steps:
   • Modify the Post model to include the new column.
   • Create a new migration script.
   • Apply the migration to the database.

1. Modify the Post model:

   class Post(db.Model):
       id = db.Column(db.Integer, primary_key=True)
       title = db.Column(db.String(100), nullable=False)
       content = db.Column(db.Text, nullable=False)
       created_at = db.Column(db.DateTime, default=db.func.current_timestamp())
       updated_at = db.Column(db.DateTime, onupdate=db.func.current_timestamp())
   

2. Create a new migration script:

   flask db migrate -m "Add updated_at column to Post model"
   

3. Apply the migration:

   flask db upgrade
   

• Forgetting to Apply Migrations: Always remember to run flask db upgrade after creating a migration script.
• Not Initializing Flask-Migrate: Ensure you have initialized Flask-Migrate in your application before running migration commands.
• Descriptive Migration Messages: Use descriptive messages when creating migration scripts to keep track of changes easily.

In this section, you learned how to set up and use Flask-Migrate to manage database migrations in your Flask application. You now know how to create and apply migration scripts, as well as handle changes to your database schema over time. This knowledge is crucial for maintaining and evolving your application's database structure efficiently.