You mean you want to use OO in Java? Then you need Twig
http://code.google.com/p/twig-persist/
It supports polymorphic relations and inheritance just like your
example with no configuration
For example you could have:
class D
{
Collection<C> cs;
}
which could contain A's or B's.
To speed up querying and loading D's you can define them to be
embedded as components:
class D
{
@Component(polymorphic=true) Collection<C> cs;
}
The polymorphic flag is needed to tell Twig to store the actual class
of the embedded component as an extra property which it needs a read-
time to create the instance. Usually this is not needed because Twig
can use the generic type of the Field to know that the Collection is
of C's.
JD
On Jan 15, 3:59 am, Federico Keen <[email protected]> wrote:
> Hi guys, I'm trying to use the datastore with a model like this:
>
> Class A extends C
> {
> �...@persistent
> private String a1;
>
> }
>
> Class B extends C
> {
> �...@persistent
> private String b1;
> �...@persistent
> private D dAttribute;
>
> }
>
> Class D
> {
> �...@persistent
> private String d1;
>
> }
>
> Class C
> {
> �...@primarykey
> �...@persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> private Key id;
>
> �...@persistent
> private double c1;
> �...@persistent
> private double c2;
>
> }
>
> How should I declare this in Java?
> How can I make queries? The following queries are failing and I don't
> can't find more help!
>
> public List<A> get( String minC1, String maxC1 ) {
> PersistenceManager pm = WherePersistanceManagerFactory.get
> ().getPersistenceManager();
> Query query = pm.newQuery(A.class);
> query.setFilter("c1 >= " + minC1 + " && c1 <= " + maxC1);
> return (List<A>) query.execute();
> }
>
> public List<B> get( String minC1, String maxC1 ) {
> PersistenceManager pm = WherePersistanceManagerFactory.get
> ().getPersistenceManager();
> Query query = pm.newQuery(B.class);
> query.setFilter("c1 >= " + minC1 + " && c1 <= " + maxC1 + " &&
> dAttribute.d1 = someConstant");
> return (List<B>) query.execute();
> }
> I can't understand how to store/retrieve classes that inherit from
> other classes or classes as attributes of other classes. I always get
> an error saying that the subclass hasn't got the superclass attribute
> or that the class that has a reference to the other class cannot find
> that attribute.
> Can anyone help me?
> Thanks!
--
You received this message because you are subscribed to the Google Groups
"Google App Engine for Java" group.
To post to this group, send email to [email protected].
To unsubscribe from this group, send email to
[email protected].
For more options, visit this group at
http://groups.google.com/group/google-appengine-java?hl=en.
