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In this chapter you will learn to use Flask-Migrate to safely manage database schema changes and understand the corresponding relationship with Django migrations. * * * ## Why can't we always use db.create_all()? `db.create_all()` has two fatal flaws: * It only creates tables if they don't exist. When modifying models (such as adding new fields), existing tables will not be updated * It doesn't preserve change history. Cannot rollback to previous database states Database migration is like Git for databases — each schema change generates a version file, supporting upgrades and rollbacks. Flask-Migrate is a Flask extension for Alembic, designed specifically for SQLAlchemy. * * * ## Installation and Configuration (venv) $ pip install flask-migrate ## Example # File path: app.py from flask import Flask from flask_sqlalchemy import SQLAlchemy from flask_migrate import Migrate app = Flask( __name__ ) app.config['SQLALCHEMY_DATABASE_URI']='sqlite:///blog.db' db = SQLAlchemy(app) migrate = Migrate(app, db)# Bind app and db to Migrate * * * ## The Three Steps of Migration | Step | Command | Function | Django Equivalent | | --- | --- | --- | --- | | Initialize | flask db init | Create migrations/ directory (only once) | — | | Generate | flask db migrate -m "description" | Detect model changes, generate migration script | makemigrations | | Execute | flask db upgrade | Apply migration script to database | migrate | ### Step 1: Initialize Migration Directory (venv) $ flask db init Creating directory migrations/... done After execution, the `migrations/` directory is created (similar to Git's .git directory), only needs to be executed once. ### Step 2: Generate Migration File (venv) $ flask db migrate -m "Initialize Post and Category models" INFO [alembic.runtime.migration] Context impl SQLiteImpl.Generating migrations/versions/xxxx_initial.py ... done If you haven't made any model changes yet, you can directly run this command on existing models. ### Step 3: Execute Migration (venv) $ flask db upgrade INFO [alembic.runtime.migration] Running upgrade -> xxxx, initialize * * * ## Practical: Add a New Field to Post Model Assume product requirements change and you need to add a read count to articles. ## Example # Add to Post class in models.py class Post(db.Model): # ... existing fields ... read_count = db.Column(db.Integer, default=0)# New: read count After modifying the model, execute migration: (venv) $ flask db migrate -m "Add read_count field to Post model"(venv) $ flask db upgrade Now the `posts` table in the database automatically has the `read_count` column added, and existing data is not affected. ### View Migration History View which version the current database is at: (venv) $ flask db current View all migration history: (venv) $ flask db history * * * ## Rollback Migration Migrations can be reverted: (venv) $ flask db downgrade # Rollback to previous version This will delete the `read_count` column and revert all changes from the last migration. > Not all operations can be automatically rolled back (such as deleting columns in SQLite). Before rolling back, first confirm with `flask db current` to view the current version, ensure you know what you are reverting. Be sure to backup data before rolling back in production environment. * * * ## Flask-Migrate vs Django Migration Comparison | Operation | Flask (Flask-Migrate) | Django | | --- | --- | --- | | Initialize | flask db init | Not needed (project has migrations directory built-in) | | Generate migration | flask db migrate -m "description" | python manage.py makemigrations | | Execute migration | flask db upgrade | python manage.py migrate | | Rollback | flask db downgrade | python manage.py migrate app 0001 | | View history | flask db history | python manage.py showmigrations | | Automation level | Requires initialization, depends on Alembic | Fully automated, no additional configuration needed | Django's migration system is more automated and integrated; Flask achieves the same level of capability through Flask-Migrate. * * * ## Chapter Summary In this chapter you mastered the complete Flask-Migrate process: `flask db init` for initialization, `flask db migrate` to generate migration scripts, `flask db upgrade` to execute migrations, and `flask db downgrade` to rollback. From now on, every time you modify models.py, follow this three-step process, and database schema changes will always have a traceable history.