Don’t Generate Empty Migrations with Autogenerate

不要使用 Autogenerate 生成空迁移

A common request is to have the alembic revision --autogenerate command not actually generate a revision file if no changes to the schema is detected. Using the EnvironmentContext.configure.process_revision_directives hook, this is straightforward; place a process_revision_directives hook in MigrationContext.configure() which removes the single MigrationScript directive if it is empty of any operations:

一个常见的要求是如果没有检测到 schema 的更改，则让 alembic revision --autogenerate 命令不实际生成修订文件。 使用 EnvironmentContext.configure.process_revision_directives 钩子，这很简单； 在 MigrationContext.configure() 中放置一个 process_revision_directives 钩子，如果单个 MigrationScript 指令没有任何操作，它将删除它：

def run_migrations_online():

    # ...

    def process_revision_directives(context, revision, directives):
        if config.cmd_opts.autogenerate:
            script = directives[0]
            if script.upgrade_ops.is_empty():
                directives[:] = []


    # connectable = ...

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata,
            process_revision_directives=process_revision_directives
        )

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