Operation Reference

操作参考

This file provides documentation on Alembic migration directives.

此文件提供有关 Alembic 迁移指令的文档。

The directives here are used within user-defined migration files, within the upgrade() and downgrade() functions, as well as any functions further invoked by those.

此处的指令用于 用户定义的迁移文件、upgrade() 和 downgrade() 函数以及由这些函数进一步调用的任何函数。

All directives exist as methods on a class called Operations. When migration scripts are run, this object is made available to the script via the alembic.op datamember, which is a proxy to an actual instance of Operations. Currently, alembic.op is a real Python module, populated with individual proxies for each method on Operations, so symbols can be imported safely from the alembic.op namespace.

所有指令都作为称为 Operations 的类上的方法存在。 运行迁移脚本时，该对象通过 alembic.op 数据成员对脚本可用，该数据成员是操作的实际实例的代理。 目前，alembic.op 是一个真正的 Python 模块，为操作上的每个方法填充了单独的代理，因此可以从 alembic.op 命名空间安全地导入符号。

The Operations system is also fully extensible. See Operation Plugins for details on this.

Operations系统 也是完全可扩展的。 有关这方面的详细信息，请参阅 Operation Plugins 。

A key design philosophy to the Operation Directives methods is that to the greatest degree possible, they internally generate the appropriate SQLAlchemy metadata, typically involving Table and Constraint objects. This so that migration instructions can be given in terms of just the string names and/or flags involved. The exceptions to this rule include the add_column() and create_table() directives, which require full Column objects, though the table metadata is still generated here.

Operation Directives 方法的一个关键设计理念是尽可能地在内部生成适当的 SQLAlchemy 元数据，通常涉及 Table 和 Constraint 对象。 这样就可以仅根据所涉及的字符串名称和/或标志给出迁移指令。 此规则的例外情况包括 add_column() 和 create_table() 指令，它们需要完整的 Column 对象，但仍会在此处生成表元数据。

The functions here all require that a MigrationContext has been configured within the env.py script first, which is typically via EnvironmentContext.configure(). Under normal circumstances they are called from an actual migration script, which itself would be invoked by the EnvironmentContext.run_migrations() method.

这里的函数都需要首先在 env.py 脚本中配置 MigrationContext，这通常是通过 EnvironmentContext.configure()。 在正常情况下，它们是从实际的迁移脚本中调用的，该脚本本身将由 EnvironmentContext.run_migrations() 方法调用。