Database Migrations in Flask: Step-by-Step Guide

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Introduction

In this article, we will explore how to use Flask-Migrate for schema changes, as well as applying and managing migrations in a Flask application.

Step 1: Setting Up Flask and Flask-Migrate

First, ensure you have Flask and Flask-Migrate installed. You can install them using pip:

            pip install flask flask_sqlalchemy flask-migrate
        

Set up your Flask application and configure SQLAlchemy:

            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)
        

Step 2: Define a Model

Create a simple model, such as a User model:

            class User(db.Model):
                id = db.Column(db.Integer, primary_key=True)
                name = db.Column(db.String(100), nullable=False)
                email = db.Column(db.String(120), unique=True, nullable=False)
        

At this stage, your database is not yet created. Flask-Migrate will handle this.

Step 3: Initialize Flask-Migrate

Initialize Flask-Migrate in your project by running the following command in the terminal:

            flask db init
        

This creates a migrations folder to track migration files.

Step 4: Create the First Migration

Generate the migration script to create the User table:

            flask db migrate -m "Initial migration"
        

This creates a new migration file in the migrations/versions folder.

Step 5: Apply the Migration

Apply the migration to the database using the following command:

            flask db upgrade
        

This creates the User table in your database.

Step 6: Modifying the Schema

To demonstrate schema changes, add a new column to the User model:

            class User(db.Model):
                id = db.Column(db.Integer, primary_key=True)
                name = db.Column(db.String(100), nullable=False)
                email = db.Column(db.String(120), unique=True, nullable=False)
                age = db.Column(db.Integer)  
        

Generate a new migration to reflect this change:

            flask db migrate -m "Add age column to User"
        

Apply the migration to update the database schema:

            flask db upgrade
        

Step 7: Managing Migrations

To view the current state of migrations, use:

            flask db history
        

To downgrade to a previous migration, use:

            flask db downgrade
        

For example, to downgrade one step:

            flask db downgrade -1
        

Step 8: Rollback Changes

If you need to undo a migration completely, you can use the downgrade command to revert all changes and then remove the migration files.

            flask db downgrade base
        

After downgrading, you can delete the migration files in the migrations folder.

Conclusion

Flask-Migrate simplifies database schema changes by providing tools to generate and manage migrations. By following these steps, you can efficiently handle database changes in your Flask projects.

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