Equivalent of a repeated NDB StructuredProperty in JPA/JDO on Google App Engine

Question

A list of StructuredProperties in NDB is modelled by a number of repeated properties, one repeated property per property of the StructuredProperty's model class. (See here: https://developers.google.com/appengine/docs/python/ndb/properties#structured.)

The closed equivalent I have found with JPA on Google App Engine is an @Embedded list of @Embeddables. The storage format, however, is now different. The GAE/J plugin for JPA generates one column per embedded entity per property of the embedded entity (see here: http://datanucleus-appengine.googlecode.com/svn-history/r979/trunk/src/com/google/appengine/datanucleus/StoreFieldManager.java). (For long lists, this generates rows with many, many columns, for example.)

Is there an easy built-in way to copy NDB's way to cope with repeated composite values using JPA or JDO?

ADDED:

For those more comfortable with Java than Python, I have found that the Java-based Objectify framework stores collections of embedded classes in the same way as NDB on Python: http://code.google.com/p/objectify-appengine/wiki/Entities#Embedding, which is the way I want to achieve with JPA (or JDO):

For convenience, I am copying their example here:

import com.googlecode.objectify.annotation.Embed;
import com.googlecode.objectify.annotation.Entity;
import com.googlecode.objectify.annotation.Id;

@Embed
class LevelTwo {
    String bar;
}

@Embed
class LevelOne {
    String foo;
    LevelTwo two
}

@Entity
class EntityWithEmbeddedCollection {
    @Id Long id;
    List<LevelOne> ones = new ArrayList<LevelOne>();
}

Using this setup, run the following code:

EntityWithEmbeddedCollection ent = new EntityWithEmbeddedCollection();
for (int i=1; i<=4; i++) {
    LevelOne one = new LevelOne();
    one.foo = "foo" + i;
    one.two = new LevelTwo();
    one.two.bar = "bar" + i;

    ent.ones.add(one);
}

ofy().save().entity(ent);

The resulting row layout (using repeated properties on the Datastore level) is then:

ones.foo: ["foo1", "foo2", "foo3", "foo4"]

ones.two.bar: ["bar1", "bar2", "bar3", "bar4"]

define the java class(es) rather than referring to some Python syntax. When that is done then someone can respond on how to persist it using JDO or JPA — DataNucleus
Added some code how it could be done with Objectify (which needs no mention of Python). — Marc

DataNucleus DataNucleus · Accepted Answer · 2013-04-18T09:21:19

Google's JDO/JPA plugin definition of embedded collections was specified in https://code.google.com/p/datanucleus-appengine/issues/detail?id=258&can=1&q=embedded&colspec=ID%20Stars%20Type%20Status%20Priority%20FoundIn%20TargetRelease%20Owner%20Summary

If you want some other definition of how that is stored (and there are many many ways in which it could be stored) then you raise an issue on that issue tracker of Googles (and if you want the feature sooner rather than later, then provide some resource to implement it)

Equivalent of a repeated NDB StructuredProperty in JPA/JDO on Google App Engine

1 Answers