Question

AppEngine only supports “TABLE_PER_CLASS” and “MAPPED_SUPERCLASS” for JPA inheritance.
Unfortunately “JOINED” and especially “SINGLE_TABLE” are not supported.

I’m wondering what the best alternative is to implement a SINGLE_TABLE alternative?

My only requirements are:
1) Have separate classes like AbstractEmployee, InternalEmployee, ExternalExmployee.
2) Being able to run a query over all employees, thus resulting in both InternalEmployee and ExternalEmployee instances.

The only thing I’m thinking off is using a ‘big’ Employee object containing all fields?
Any other ideas?

PS: vote for proper “SINGLE_TABLE” support via http://code.google.com/p/googleappengine/issues/detail?id=8366

Answer 1

You could in theory use @Embeded and @Embeddable to group related fields into an object. So you would have a class that looks something like.

@Entity
public class Employee {
     // all the common employee fields go here 
     //

     // the discriminator column on Employee class lets you be specific in your queries          
     private Integer type; 

     @Emebded 
     private Internal internal; // has the fields that are internal

     @Embeded
     private External external; // has the fields that are external 

     equals & hashcode that compare based on the discriminator type and other fields 
}