**Course Title:** Mastering Flask Framework: Building Modern Web Applications **Section Title:** Working with Databases: SQLAlchemy **Topic:** Creating and migrating databases using Flask-Migrate **Overview** In the previous topic, we introduced SQLAlchemy, a popular ORM tool for Python, and explored how to use it to manage databases in Flask applications. In this topic, we will take it a step further by learning how to create and migrate databases using Flask-Migrate, a extension that adds migration support to Flask-SQLAlchemy. **Why Database Migration?** Database migration is an essential process in web development that allows us to manage changes to our database schema over time. As our application evolves, we may need to add new tables, modify existing ones, or remove unused columns. Without a proper migration strategy, these changes can lead to data inconsistencies and application downtime. **What is Flask-Migrate?** Flask-Migrate is an extension that adds migration support to Flask-SQLAlchemy. It allows us to manage database schema changes by creating and applying migrations, which are essentially version-controlled changes to our database structure. **Installing Flask-Migrate** To get started with Flask-Migrate, we need to install it using pip: ```bash pip install flask-migrate ``` **Initializing Flask-Migrate** Once installed, we need to initialize Flask-Migrate in our Flask application. We can do this by creating an instance of the `Migrate` class and passing our Flask application and SQLAlchemy database instances: ```python from flask import Flask from flask_sqlalchemy import SQLAlchemy from flask_migrate import Migrate app = Flask(__name__) app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///example.db" db = SQLAlchemy(app) migrate = Migrate(app, db) ``` **Creating and Applying Migrations** To create a migration, we can use the `flask db init` command to initialize the migration repository, followed by `flask db migrate` to create a new migration script: ```bash flask db init flask db migrate ``` This will create a new migration script in the `migrations/versions` directory. We can then apply the migration using the `flask db upgrade` command: ```bash flask db upgrade ``` **Understanding Migration Scripts** Migration scripts are used to define the changes to be made to our database schema. They typically consist of two functions: `upgrade` and `downgrade`. The `upgrade` function defines the changes to be made when applying the migration, while the `downgrade` function defines the changes to be made when reverting the migration. Here's an example migration script that adds a new column to an existing table: ```python from alembic import op import sqlalchemy as sa revision = 'head' down_revision = 'base' def upgrade(): op.add_column('users', sa.Column('email', sa.String(length=100), nullable=True)) def downgrade(): op.drop_column('users', 'email') ``` **Managing Migrations** Flask-Migrate provides a range of commands to manage migrations, including: * `flask db init`: Initializes the migration repository. * `flask db migrate`: Creates a new migration script. * `flask db upgrade`: Applies pending migrations to the database. * `flask db downgrade`: Reverts the last migration applied to the database. * `flask db current`: Displays the current migration version. **Conclusion** In this topic, we learned how to create and migrate databases using Flask-Migrate. We covered the basics of database migration, installed and initialized Flask-Migrate, and explored how to create and apply migrations. By managing database schema changes effectively, we can ensure that our application remains stable and maintainable over time. **What's Next?** In the next topic, we will explore how to understand relationships and querying with SQLAlchemy. **Further Reading** * [Flask-Migrate Documentation](https://flask-migrate.readthedocs.io/en/latest/) * [Alembic Documentation](https://alembic.readthedocs.io/en/latest/) **Do you have any questions or need help with this topic? Leave a comment below!**