DPL and composite pattern

Question

Hello...using DPL, JE (3.1.0), and trying to come up with a design that fits the following mold:
@Entity
public class Thing {
   @PrimaryKey
   long id;
   @SecondaryKey(relate=MANY_TO_ONE)
   int thingTypeId;
   // how to link this to thing attribute model below?
   List<ThingAttribute> thingAtts = new ArrayList<ThingAttribute>( );
}Thus, a Thing can have a list of attributes. A thing has a unique id (primary key), and a thing has a thing-type id...and there can be many instances of that thing-type, so it is MANY_TO_ONE.
Now, a ThingAttribute follows the Composite Pattern...as below. I took a stab at annotations here, but I'm not sure it seems correct:
@Persistent
public abstract class AbstractThingAttribute {
   /* only simple attribute subclass has an id -- composite subclass does not
    * so we add a sequence here
   @PrimaryKey(sequence="ID")
   long id;
   // thing type id
   @SecondaryKey(relate=MANY_TO_ONE)
   int thingAttributeType;
@Entity
public class SimpleThingAttribute<T> extends AbstractThingAttribute {
  // unique id
  @SecondaryKey(relate=ONE_TO_ONE)
  long id;
  // generic type value
  T value;
@Entity
public class CompositeThingAttribute extends AbstractThingAttribute {
  // this is a recursive-definition relationship; not sure how to deal with DPL?
  // list of children
  List<ThingAttribute> children = new ArrayList<ThingAttribute>( );
}As an aside, there is an interface for ThingAttribute, SimpleThingAttribute, and CompositeThingAttribute...this is not too relevant for this example, but Simple/Composite interfaces extend the ThingAttribute interface.
Q1) If I create a new Thing object, and I want to persist it, it is clear to me how it works for the primary secondary key here, but how do I annotate the ThingAttribute hierarchy such that it gets persisted along with the Thing object? I guess this boils down to a couple things:
a) how to treat the thingAtts ArrayList in a Thing object itself so it gets persisted?
b) now, i have a list of objects, where each entry in the list follows the structure of the composite pattern. how should i approach annotating this model?
Thanks in advance,
Davis

Answer 1

Hi Davis!
Q1) If I create a new Thing object, and I want to
persist it, it is clear to me how it works for the
primary secondary key here, but how do I annotate the
ThingAttribute hierarchy such that it gets persisted
along with the Thing object? I guess this boils down
to a couple things:
a) how to treat the thingAtts ArrayList
in a Thing object itself so it gets persisted?You just embed the thingAttrs object (as you have) in the entity. As long as the class is @Persistent (or a simple type) you can persist arbitrarily complex graphs of objects. The thin to remember is that it is stored as one object graph per entity, indexed by primary key and optionally by one or more secondary keys.
b) now, i have a list of objects, where each entry
in the list follows the structure of the composite
pattern. how should i approach annotating this
model? Don't annotate the embedded objects with @Entity, just with @Persistent. @Entity is only used when you want a separate PrimaryIndex. Objects embedded inside an entity are annotated with @Persistent only.
So when you have multiple objects you can use either embedding or relationships. With embedding, the object graph for the entire entity is stored in a single record.
With relationships, you store the keys of one entity as a field in the other entity, and you annotate that field with @SecondaryIndex(relate=..., relatedEntity=...). With relationships, you have to get the related object yourself via its own PrimaryIndex.
Hope this helps,
Mark