Sharing a Connection with a Series of Migration Commands and Environments

与一系列迁移命令和环境共享连接

It is often the case that an application will need to call upon a series of commands within Commands, where it would be advantageous for all operations to proceed along a single transaction. The connectivity for a migration is typically solely determined within the env.py script of a migration environment, which is called within the scope of a command.

通常情况下，应用程序需要调用 Commands 中的一系列命令，在这种情况下，所有操作都沿单个事务进行是有利的。 迁移的连接性通常仅在迁移环境的 env.py 脚本中确定，该脚本在命令范围内调用。

The steps to take here are:

这里要采取的步骤是：

  1. Produce the Connection object to use.
  2. Place it somewhere that env.py will be able to access it. This can be either a. a module-level global somewhere, or b. an attribute which we place into the Config.attributes dictionary (if we are on an older Alembic version, we may also attach an attribute directly to the Config object).
  3. The env.py script is modified such that it looks for this Connection and makes use of it, in lieu of building up its own Engine instance.
  1. 生成要使用的 Connection 对象。
  2. 将它放在 env.py 能够访问它的地方。 这可以是全局的模块级的 a.a 或 b.，然后我们将其作为属性放入 Config.attributes 字典。（如果我们使用的是较旧的 Alembic 版本，我们也可以将属性直接附加到 Config 对象）。
  3. 修改了 env.py 脚本，使其查找此 Connection 并使用它，而不是构建自己的 Engine 实例。

We illustrate using Config.attributes:

我们使用 Config.attributes 进行说明：

from alembic import command, config

cfg = config.Config("/path/to/yourapp/alembic.ini")
with engine.begin() as connection:
    cfg.attributes['connection'] = connection
    command.upgrade(cfg, "head")

Then in env.py:

然后在 env.py 中：

def run_migrations_online():
    connectable = config.attributes.get('connection', None)

    if connectable is None:
        # only create Engine if we don't have a Connection
        # from the outside
        connectable = engine_from_config(
            config.get_section(config.config_ini_section),
            prefix='sqlalchemy.',
            poolclass=pool.NullPool)

    context.configure(
        connection=connectable,
        target_metadata=target_metadata
    )

    with context.begin_transaction():
        context.run_migrations()

Changed in version 1.4: Prior to this version, we used a “branched connection”, by calling connect(). This is now deprecated and unnecessary, since we no longer have to guess if the given “connection” is an Engine or Connection, it is always a Connection.

版本1.4更新: 在此版本之前，我们通过调用 connect() 使用 “分支连接” 。 现在已弃用且不必要，因为我们不再需要猜测给定的 connection 是 Engine的实例还是Connection的实例，它始终是Connection的实例。