Question

I am currently working with GWT, GAE and using JPA as my ORM. I have an issue where the keys that GAE is generating are too large reasonably to be used on a mobile device with RequestFactory. The amount of data in a small list is overwhelming due to the size of the ID/KEY when converted to String.

I am using String for my key’s so that I can handle inheritence.

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")  
  protected String key;

This creates a key that is very long example “agxzbWFydGJhcnNpdGVyFAsSDUVzdGFibGlzaG1lbnQYuAIM” and gets larger due to storing object type and parent in the key.

I need a way to create a smaller unique id but still have the ability to handle inheritence in GAE. I tried Long as the @Id/key but was not able to use a @OneToMany relationship on my objects due to the relationship that is built into the String/Key key.

The other option is to create a sequence for each class and use a Long property for that id. There is an example below but I am not sure how to handle a generated Long sequence in app engine.

@GeneratedValue
private Long friendlyClassSpecificKey;

Any advice would be appreciated. If there is another option other than using the sequence for each class type I am interested but if not is there an example of creating a sequence (That is not the @ID) for a specific class?

Answer 1

I came up with a good solution for smaller keys. I think the best way to do this cleanly is to use jpa/jdo 2 for app engine with an unowned relationship. This way you can fetch the keys from (Long) id using just their type and not have to use the parent relationship.

This is the base datstore object and notice I am using the app engine key.

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class DatastoreObject {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Key key;

  public Long getId() {
    return key.getId();
  }

}

This class will use the @Unowned attribute supported in jpa 2 so that the inventory’s key does not contain the parent establishment key. Otherwise you would have to pass the parent id in also and resolve that to a key based on type. This is because in an owned relationship the child key contains the parent key also.

@Entity
public class Establishment extends DatastoreObject {

    @Unowned
    @OneToOne(cascade = CascadeType.ALL)
    private Inventory inventory;

}

Then in my dao base class I use the class

public class DaoBase<T extends DatastoreObject> {

protected Class<T> clazz;

    @SuppressWarnings("unchecked")
    public DaoBase() {
        clazz = (Class<T>) ((ParameterizedType) getClass()
                .getGenericSuperclass()).getActualTypeArguments()[0];
    }


    /**
     * Find an object by it's shortened long id
     * @param id
     * @return
     * @throws EntityNotFoundException
     */
    public T find(Long id) {
        if (id == null) {
            return null;
        }
        EntityManager em = ThreadLocalPersistenceManager.getEntityManager();

        Key key = getKey(id);

        T obj = em.find(clazz, key);
        return obj;
    }

    protected Key getKey(Long id) {
        return KeyFactory.createKey(clazz.getSimpleName(), id);
    }
}