Question

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

Answer 1

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 Foo and you suddenly add a property, prop = db.BooleanProperty(default=True) to it, all existing Foo entities will not have prop set to True (though new ones will). Likewise, you will have to manage ReferencePropertys and ListProperty(db.Key) manually. App Engine does have a _set operator 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 _set functionality.