Migrations with Flask-Migrate

Key Concepts

• Database Migrations
• Flask-Migrate
• Migration Commands
• Schema Changes
• Version Control for Databases

Database Migrations

Database migrations are a way to manage changes to the database schema over time. They allow you to evolve your database schema in a controlled and organized manner, ensuring that all instances of your application use the correct schema.

Flask-Migrate

Flask-Migrate is an extension for Flask that handles SQLAlchemy database migrations. It integrates with Alembic, a lightweight database migration tool, to provide a simple interface for managing database schema changes.

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'

db = SQLAlchemy(app)
migrate = Migrate(app, db)
    

Migration Commands

Flask-Migrate provides several commands to manage migrations. These commands include creating a new migration, upgrading the database to the latest version, and rolling back to a previous version.

# Initialize the migration repository
flask db init

# Create a new migration script
flask db migrate -m "Initial migration"

# Apply the migration to the database
flask db upgrade

# Rollback to the previous version
flask db downgrade
    

Schema Changes

Schema changes involve modifying the structure of your database tables. This can include adding new tables, modifying existing tables, or deleting tables. Migrations help manage these changes by creating scripts that describe the changes to be applied.

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)

# After adding a new column
class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    age = db.Column(db.Integer, nullable=True)
    

Version Control for Databases

Just as code is version-controlled using tools like Git, database schema changes can be version-controlled using migrations. This ensures that all developers and environments use the same database schema, reducing errors and inconsistencies.

# Example of a migration script
def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.add_column('user', sa.Column('age', sa.Integer(), nullable=True))
    # ### end Alembic commands ###

def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('user', 'age')
    # ### end Alembic commands ###