How the existing data to be if entity structure modified or deleted on GAE?

GAE recommends using JDO/JPA. But I have serious question about using OODB like them. JDO based on user's class structure. And d开发者_如何学Goata structure should be modified continually as service advances. So,

1. If data(entity) class property being removed, what happened to existing data on the property?
2. If data(entity) class renamed for refactoring reason, how the JDO know those renaming? Or all data loss?

Major point is "How JDO/GAE/BigTable applies modification of application level structure(class/attributes) into existing data on DB?".

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Take a look at Updating Your Model's Schema.

1. Existing entities will still have the old property. New entities will not.
2. You can map a new entity class to an existing Kind in the datastore. However, if you want to change the name of the Kind in the datastore then that's not as easy.