Persisting @ElementCollection of @Embeddable (Google App Engine, datanucleus)

Question

I am trying to persist a JPA entity with a collection of custom @Embeddable objects using the JPA2 @ElementCollection annotation. Simple example (both classes are enhanced by datanucleus):

@Entity
public class TestEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ElementCollection
    private Set<TestEmbeddable> testEmbeddables;

    public Set<TestEmbeddable> testEmbeddables() {
        return this.testEmbeddables;
    }
}

@Embeddable
public class TestEmbeddable implements Serializable {
    public String s;
}

The persisted Datastore entity, however, will only contain a collection of null values instead of the actual objects:

TestEntity.testEmbeddables = [null, null, ...]

Persisting a collection of basic types such as Strings or embedding a single TestEmbeddable object using @Embedded works perfectly fine. Would someone be able to clarify whether element collections of embeddables are supported by datanucleus-appengine?

While the datanucleus section on JPA element collections only gives an example for a String collection, the corresponding JDO section uses a custom embedded-only type. The feature list further states that embedded collections in general are compatible with GAE, but does not say whether custom types are supported. I also found one other person claiming that this should work.

-- Edit --

Following DataNucleus' answer, I ran some more tests:

@ElementCollection
private List<String> stringsElementCollection;

--> Works. The individual Strings are persisted as TestEntity.stringsElementCollection = [str1, str2, ...]

@Embedded
private List<String> stringsEmbedded;

--> Same as @ElementCollection. I wonder if the JPA specification covers the use of @Embedded on a collection, though?

@ElementCollection
private List<TestEmbeddable> embeddablesElementCollection;

--> Doesn't work. Instead of the actual TestEmbeddable objects, the Datastore persists only a collection of null values: TestEntity.embeddablesElementCollection = [null, null, ...]

@Embedded
private List<TestEmbeddable> embeddablesEmbedded;

--> This seems to work. The TestEmbeddable.s field is stored as TestEntity.s.0, .s.1, etc. plus a TestEntity.embeddablesEmbedded.size property.

(App Engine SDK 1.7.7.1, datanucleus 3.1.3, datanucleus-appengine 2.1.2)

Have you tried code.google.com/p/objectify-appengine. This API encapsulates lot of things for your development. — sathish_at_madison

DataNucleus DataNucleus · Accepted Answer · 2013-04-24T06:45:41

No idea if Google support embedded collections of such objects yet with JPA. That was only developed for JDO and works there. You could try putting a @Embedded on the embedded field to reinforce the idea that its embedded.

Persisting @ElementCollection of @Embeddable (Google App Engine, datanucleus)

2 Answers