SoFunction
Updated on 2025-03-02

Flask creates and runs database migration implementation process

1. Preparation

1. Install the necessary packages

First, make sure that Flask is installed along with Flask-SQLAlchemy (for database operations) and Flask-Migrate (for database migration). If it has not been installed, you can install it through the pip command:

pip install Flask Flask-SQLAlchemy Flask-Migrate

2. Configure the database

In Flask applications, you need to configure database connections. This is usually done in the application's configuration file, e.g.. The configuration content may include the database type (such as SQLite, MySQL, PostgreSQL, etc.), username, password, host address, port number, and database name.

class Config:  
    # Example configuration, taking MySQL as an example    SQLALCHEMY_DATABASE_URI = 'mysql://username:password@host:port/dbname'  
    SQLALCHEMY_TRACK_MODIFICATIONS = False  # Disable tracking of object modifications and send signals

Then, in the main file of the Flask application (e.g.) Import and apply this configuration.

3. Initialize SQLAlchemy and Migrate

In Flask applications, SQLAlchemy and Migrate need to be initialized. This is usually done in the main file of the application.

from flask import Flask  
from flask_sqlalchemy import SQLAlchemy  
from flask_migrate import Migrate  
  
app = Flask(__name__)  
.from_object('')  # Assume that the configuration file is named and the configuration class name is Config  
db = SQLAlchemy(app)  
migrate = Migrate(app, db)

2. Create a database migration

1. Define the model

In Flask applications, the model is usually defined inin the file. These models represent tables in the database and use SQLAlchemy's ORM (Object Relationship Mapping) function to define the fields and relationships of the table.

#   
from app import db  
  
class User():  
    id = (, primary_key=True)  
    username = ((80), unique=True, nullable=False)  
    email = ((120), unique=True, nullable=False)  
  
    def __repr__(self):  
        return '<User %r>' %

2. Initialize the migration warehouse

In the terminal, go to the root directory of the Flask application and run the following command to initialize the migration repository. This command will create a name calledmigrationsfolder to store all migration files.

flask db init

Note: If you have not added Flask's command line interface (CLI) to your application, you may need to call it directly.flask_migratecommand line tool, or throughpython -m flask db initrun in the way.

3. Generate migration scripts

After modifying the model, a migration script needs to be generated that describes the changes from the current database schema to the updated database schema.

flask db migrate -m "Add User Model"

Here-mThe option allows you to specify a message for the migration that will appear at the head of the migration file to indicate the purpose or content of the migration.

4. Review the migration script

The generated migration script will be saved inmigrationsIn the folder, the file name is usually similarversion_xxxx_migration_message.pyformat. Before applying these changes to the database, it is recommended to review these scripts to make sure they correctly reflect your model changes.

3. Run database migration

1. Application migration

Once you are satisfied with the content of the migration script, you can apply the migration to the database through the following command.

flask db upgrade

This command looks for the latest migration script and applies it to the database. If the database schema is already up to date, this command will not perform any operations.

2. Downgrade migration

If you need to roll back to the previous database version, you can usedowngradeOrder.

flask db downgrade

By default,downgradeThe command will roll back to the previous migration version. If you need to roll back to a specific version, you can use--revisionOptions specify the target version.

4. Other precautions

  1. Database backup: Before performing any migration, it is recommended to back up the database in case of unforeseen problems during the migration process.
  2. Migration script management: Over time, there may be many migration scripts in your application. It is recommended to review these scripts regularly to ensure they are still valid and to remove any scripts that are no longer needed.
  3. Migration dependencies: If your application depends on a specific database feature or version, make sure to consider these dependencies in the migration script.
  4. test: Before applying migrations to production databases, run them in a development or test environment to make sure they work as expected.

By following the above steps, you can effectively create and run database migrations in your Flask application, thereby managing changes in database schemas and ensuring consistency and integrity of your data. While this process may involve some complexity and challenges, it provides strong support and flexibility for database management during development.

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