Entity hierarchies in Objectify 4.0

问题

Is there a way how to define an entity hierarchy that enables to query just particular subclass? Consider situation below. Let's have abstract Base class that defines common properties and concrete subclasses A and B.

class abstract Base {
...
}

class A extends Base {
...
}

class B extends Base {
...
}

I would like to run for example queries as follows.

• To retrieve all entities of type A and B

   Base base = this.objectify.load().type(Base.class).list();
  

• To retrieve all entities of type A

   Base base = this.objectify.load().type(A.class).list();
  

• To retrieve all entities of type B

   Base base = this.objectify.load().type(B.class).list();
  

Furthermore, we would like to store all such entities as a single type (Base entity) in GAE Datastore.

We tried to use polymorphic hierarchy of related entity classes described here:

https://code.google.com/p/objectify-appengine/wiki/Entities#Polymorphism

But it seems that this is not capable of handling a situation where there are multiple entity subclasses with a common parent.

回答1:

I don't think Base can be abstract, but this should work:

@Entity
class Base { ... }

@EntitySubclass(index=true)
class A extends Base { ... }

@EntitySubclass(index=true)
class B extends Base { ... }

If you want to be able to query by polymorphic type, you must index the types that you want to query by.

来源：https://stackoverflow.com/questions/19210206/entity-hierarchies-in-objectify-4-0