sqlalchemy-migrate 升级降级openstack数据库

参考 https://sqlalchemy-migrate.readthedocs.org/en/latest/versioning.html#writing-for-a-specific-database
 0.作用
 可对数据库进行版本升级/降级
 新版本的代码，数据库表结构发生变化。采用migrate可直接改变数据库的表结构而不影响原有的数据。
 一。 SQLALchemy 迁移提供两种方式
 1)api方式 migrate.versioning API
 2)migrate 命令行方式
 二。涉及概念 
 repository : 数据库迁移所需要的配置文件、脚本等保存的目录
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 三。迁移的工作流程
 命令行方式 
 1.建立repository 
 
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在创建新的repository（/home/cloud/your_repository），名为'example project'
 
 目录下的内容
 
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