Google App Engine、BigTable 和模型演变
我目前正在使用 Spring/JPA/Google App Engine，有一个问题令我担心。
GAE 的一个很酷的事情是，一旦定义了映射，我只需要插入数据，与之相关的所有内容也会被存储。
但是，如果我碰巧更改了映射，我该如何处理以前的数据呢？每次发生更改时我都必须创建迁移脚本吗？在这种情况下有没有办法使用 Liquibase 或类似的东西？或者是否有另一种方法来处理现有数据的这些更改？
非常感谢您的帮助！ 罗尔夫
I am currently playing with Spring/JPA/Google App Engine, and there is an issue that I am worrying about.
The cool thing with GAE is, once my mapping is defined, I just need to insert data and everything associated to it is stored too.
However, if I happen to change my mapping, how can I do with my previous data ? Do I have to create migration scripts each time something has changed ? Is there a way to use Liquibase or something like that in this case ? Or is there another way to handle these changes on existing data ?
Thanks a lot for your help!
Rolf
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这取决于这些更改的性质，但在大多数情况下，至少需要进行一些轻微的更新。要记住的一件大事是父子关系（请参阅 上一个问题了解更多详细信息）和设计时的实体组；由于父母成为实体键的一部分，因此它们非常不可变。与键名相同。
另一件事是应用程序引擎实体是无模式的；例如，如果您有某个类
Foo，并且突然向其添加属性prop = db.BooleanProperty(default=True)，则所有现有的Foocode> 实体不会将prop设置为 True（尽管新实体会设置为 True）。同样，您必须手动管理ReferenceProperty和ListProperty(db.Key)。 App Engine 确实有一个_set运算符来帮助解决此问题，但说实话，我发现它有点不可靠，当我知道时_set会显示为空事实上，他们不应该这样。无论如何，这里有关于使用_set功能。It depends on the nature of those changes, but in most cases, at least some light updating will be required. One big thing to keep in mind is parent-child relationships (see this previous question for more details) and entity groups while design; since parents become part of an entity key, they are very much immutable. Same with key names.
Another thing is that app engine entities are schemaless; for example if you have some class
Fooand you suddenly add a property,prop = db.BooleanProperty(default=True)to it, all existingFooentities will not havepropset to True (though new ones will). Likewise, you will have to manageReferencePropertys andListProperty(db.Key)manually. App Engine does have a_setoperator to help with this, but to be honest I've found it to be a bit unreliable, with_sets coming out empty when I knew for a fact that they should not be. Regardless, here's the documentation on using the_setfunctionality.