GhostObject.java

public class FooEntity {

    enum Status { GHOST, LOADING, LOADED };

    private Status status;
    private long id;
    private BarEntity relatedEntity;

    public FooEntity(long id) {
        this.id = id;
    }

    public BarEntity getRelatedEntity() {
        if (status == Status.GHOST)
            Datastore.load(this);
        return relatedEntity;
    }

}