Another way of solving the problem is to have a Horse object with a
"farmKey" field like so:
class Horse {
@PrimaryKey
Key mKey
Key farmKey
}
Then to get horses on a farm you'd do a query on Horse.kind with
farmKey equal to your farm key. (Keys only query would make this
faster) What's nice about this approach is that you don't need Horse
key at all, just create a Horse object, give it the farm key, and
persist it without worrying about it.
Cheers!
On Mar 18, 9:03 pm, John Patterson <[email protected]> wrote:
> On 18 Mar 2010, at 20:27, Mark wrote:
>
>
>
>
>
> > Hi,
>
> > I'm not understanding how to implement a simple unowned relationship.
> > I have two classes like:
>
> > class Farm {
> > List<Key> mHorses;
> > }
>
> > class Horse {
> > �...@primarykey
> > Key mKey;
> > }
>
> > so a Farm can have some horses. I'd like to create a new horse, then
> > put it on a farm, in one transaction.
>
> This requires Horse to be a descendant of Farm to be in the same
> entity group. Therefore a horse could not move farms unless you
> delete and recreate it with a new Key.
>
>
>
>
>
> > But how do I know the key of the
> > horse during the transaction?:
>
> > Farm farm = loadFarm(...);
> > Horse horse = new Horse();
> > horse.setName("Roger");
> > try {
> > tx = pm.currentTransaction();
> > tx.begin();
> > pm.makePersistent(horse); // is the key available now?
> > farm.mHorses.add(horse.mKey);
> > pm.makePersistent(farm);
> > tx.commit();
> > }
>
> > when is the Horse.mKey value actually set and ready for use?
>
> As soon as you "put" an entity in the datastore without a key a new
> one will be generated with a unique long value. So you can use this
> in the rest of the same transaction
>
> > Or do I
> > have to do this as two separate atomic operations:
>
> > 1) Save the horse which generates its mKey value.
> > 2) If that worked, add it to the farm, persist the farm again.
> > 3) If that worked, all good, if not, have to delete the horse cause
> > now it's in limbo?
>
> You won't need to delete any horses if it is done in a transaction -
> rollback will take care of everything.
>
> I know you are experimenting with JDO but just as an example in Twig
> to show how much cleaner it is without messing with Keys:
>
> class Farm {
> List<Horse> mHorses; // direct unowned reference
> }
>
> class Horse {
> String name;
> }
>
> No Keys are needed so your data models are actually more portable and
> natural than using JDO-GAE.
>
> Then to store a horse with the Farm as its parent go:
>
> farm.mHorses.add(myHorse);
> datastore.store(myHorse, myFarm);
> datastore.update(myFarm);
>
> The store() command creates the Key and caches it so when you update
> the farm it already has the Key.
>
> Hope some of this helps!
>
> John
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