Hi.
Well, the restraint seems to be on the way GAE handles keys. There are
4 types of keys that can be used, but the only ones that are assigned
an ID by the system is Long, Key and encodedKey.
I want the system to automatically assign an id to new objects that
are created, so that limits me to those choices.
I can't use the Long variant on child objects, so I'm stuck with the
Key/encoded Key string variants, and according to
http://code.google.com/appengine/docs/java/datastore/creatinggettinganddeletingdata.html#Keys
the system generated key is made up of the entity parent id, among
other things.
I use JDO directly, not the low level API. But the key-handling is
still dependent on GAE datastore I suppose...
/ Andreas
On Oct 14, 8:10 am, Roy Smith <roy.smith....@googlemail.com> wrote:
> It sounds like you're reading documentation for the Low Level API.
> I don't use JDO, but I suspect that JDO does not have that single-parent
> constraint.
>
> On Tue, Oct 13, 2009 at 8:49 PM, andreas_b <andreas.borg...@gmail.com>wrote:
>
>
>
> > Hi all.
>
> > I've only been working with relational databases up to now, and have
> > some problems with JDO.
>
> > I seem to have some trouble grasping the basic design patterns with
> > POJO/JDO here.
> > This is probably a very basic issue, but its hard when the mind is
> > setup to think from an entirely different perspective :-).
>
> > So, what I want to do here is to have child objects of the same type
> > referenced from different parents.
> > For example...
>
> > Say we have a class Person, class CarRegister, class Car.
>
> > A Car class with a Key.
>
> > class Car {
>
> > �...@primarykey
> > �...@persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> > private Key key;
>
> > ...other persistent objects...
>
> > }
>
> > In Person, I have a list of Car objects that the person owns.
>
> > class Person {
>
> > �...@primarykey
> > �...@persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> > private Long id;
> > �...@persistent
> > private List<Car> ownedCars;
>
> > }
>
> > In CarRegister, I also have a list of all Car objects registered.
>
> > class CarRegister{
>
> > �...@primarykey
> > �...@persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> > private Long id;
> > �...@persistent
> > private List<Car> registeredCars;
>
> > }
>
> > Is this possible? The documentation seem to suggest that a key is made
> > up of the entity parent id and some other stuff, which should mean
> > that the above wouldn't work. (The key value includes the key of the
> > entity group parent (if any) and either the app-assigned string ID or
> > the system-generated numeric ID).
>
> > I could of course save a list of keys/ids to Car objects instead, but
> > that to me would kind of defeat the purpose of this kind of database.
> > So I want a List of Car objects in both Person and CarRegister, and
> > they must somehow share the same objects in the database. This must be
> > one of the basic use cases, but I have been unable to find an optimal
> > solution for it.
>
> > Thanks in advance.
>
> > BR, Andreas
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