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Python面试题：结合Python技术，如何使用Alembic进行数据库迁移管理

news 来源：原创 2024/9/23 9:37:28

Alembic 是一个用于处理数据库迁移的轻量级工具，通常与 SQLAlchemy 一起使用。它允许你在数据库结构发生变化时，自动化地跟踪和管理这些变化。以下是一个使用 Alembic 进行数据库迁移管理的指南。

环境准备

安装 Alembic 和 SQLAlchemy:
```
pip install alembic sqlalchemy
```

创建项目结构:
假设你有一个项目结构如下：

myproject/
├── alembic/
├── myapp/
│   ├── __init__.py
│   ├── models.py
├── alembic.ini
├── main.py

初始化 Alembic

初始化 Alembic:
在项目根目录运行以下命令：
```
alembic init alembic
```
这将创建一个 alembic 目录，其中包含 env.py、script.py.mako 和 versions 目录。
配置 Alembic:
编辑 alembic.ini 文件，设置 SQLAlchemy 的数据库连接字符串。例如：
```
sqlalchemy.url = sqlite:///mydatabase.db
```

编辑 env.py:
在 alembic/env.py 中，告诉 Alembic 如何获取你的 SQLAlchemy 基础映射类。例如：

from logging.config import fileConfig
from sqlalchemy import engine_from_config
from sqlalchemy import pool
from alembic import context# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config# Interpret the config file for Python logging.
# This line sets up loggers basically.
fileConfig(config.config_file_name)# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
from myapp.models import Base
target_metadata = Base.metadata# other values from the config, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.def run_migrations_offline():"""Run migrations in 'offline' mode.This configures the context with just a URLand not an Engine, though an Engine is also acceptablehere.  By skipping the Engine creation we don't even needa DBAPI to be available.Calls to context.execute() here emit the given string to thescript output."""url = config.get_main_option("sqlalchemy.url")context.configure(url=url, target_metadata=target_metadata, literal_binds=True)with context.begin_transaction():context.run_migrations()def run_migrations_online():"""Run migrations in 'online' mode.In this scenario we need to create an Engineand associate a connection with the context."""connectable = engine_from_config(config.get_section(config.config_ini_section),prefix="sqlalchemy.",poolclass=pool.NullPool,)with connectable.connect() as connection:context.configure(connection=connection, target_metadata=target_metadata)with context.begin_transaction():context.run_migrations()if context.is_offline_mode():run_migrations_offline()
else:run_migrations_online()

创建数据库模型

在 myapp/models.py 中定义你的数据库模型：

from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmakerBase = declarative_base()class User(Base):__tablename__ = 'users'id = Column(Integer, primary_key=True, autoincrement=True)name = Column(String, nullable=False)DATABASE_URL = 'sqlite:///mydatabase.db'engine = create_engine(DATABASE_URL)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)def init_db():Base.metadata.create_all(bind=engine)

生成迁移脚本

生成迁移脚本:
当你对模型进行更改后，运行以下命令生成迁移脚本：
```
alembic revision --autogenerate -m "create users table"
```
这将在 alembic/versions 目录下生成一个新的迁移脚本。
检查并应用迁移:
生成的迁移脚本可能需要手动检查和修改。完成后，运行以下命令应用迁移：
```
alembic upgrade head
```

完整示例

以下是一个简单的完整示例：

main.py:

from myapp.models import init_dbif __name__ == "__main__":init_db()

myapp/models.py:

from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmakerBase = declarative_base()class User(Base):__tablename__ = 'users'id = Column(Integer, primary_key=True, autoincrement=True)name = Column(String, nullable=False)DATABASE_URL = 'sqlite:///mydatabase.db'engine = create_engine(DATABASE_URL)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)def init_db():Base.metadata.create_all(bind=engine)