Need to update the primary key on existing objects in GAE Java

Question

Hi,

I am building a web app using GAE Java. I have a class that uses a Long ID (generated by appengine) as its primary key.

I now want to create a new class that would be the parent class to this original class (a one to many relationship) however the child needs to have a primary key of type "key", not the Long ID I have now.

What is the best way to change the primary key to be type "key" instead of long for the existing persisted entities? Should I create a new class with primary key of type "key" and instantiate and persist new objects that copy the field values from the old ones? Or can I somehow just update the existing class?

Thanks

Answer 1

In fact, the Key of a persisted Entity is considered to be immutable. Changing the key will, without a doubt, be equivalent to changing the used instance. What I suggest you is to link your initial object to a child of you created parent.

Answer 2

You could store the existing Long IDs in a list in the parent class: that would create the necessary one-to-many parent-child relationship.

You would have to manage consistency yourself though (which may not be too difficult if your site doesn't have very high traffic), and the parent and child classes would not be in the same entity group (implications for transactions).

Changing the key means changing the Entity itself (and also its entity group).

Ultimately, the solution that works for you will depend on the specifics of your problem. For example, is there already a lot of existing data? Is this a live application (i.e., is it already being used)?

Another solution could be to migrate your app over to a different (better suited) data model, and do so account-by-account (by locking out the account for a brief period of time). This way only a few individuals will be affected by the change (if they happen to access the app when you are migrating their account) rather than the whole app being down.