One-to-many with subclasses

Question

Hello again,
there seems to be a problem with one-to-many relations involving
inheritence...
Let me describe the setting:
class AbstractUsecase {
// element-type=Account
Set accounts;
class Usecase extends AbstractUsecase {
class Account extends AbstractAccount implements InstanceCallbacks {
// element-type=AccountRevisionItem
List revisions;
AbstractUsecase uc;
class AccountRevisionItem extends Account {
long time;
User user;
As you can see a simple model (in uml it looks much more better).
In the method jdoPreStore() of Account an object of type
AccountRevisionItem will added to revisions. This new object holds the
data of Account, the User and timestamp of the modifing. So we are able to
trace every change on the date (user, date). Metadata looks like this:
<class name="AbstractUsecase"/>
<class name="Usecase" persistence-capable-superclass="AbstractUsecase">
<class name="Abstractaccount"/>
<class name="Account" persistence-capable-superclass="AbstractAccount">
<extension vendor-name="kodo" key="table" value="Account"/>
<field name="revisions">
<collection element-type="AccountRevisionItem"/>
</field>
</class>
<class name="AccountRevisionItem"
persistence-capable-superclass="Account"/>
When i get a extent of all Accounts i can fetch all revisions of each
Account.
Now the problem: When i fetch all accounts of a usecase there are objects
included of type Account AND AccountRevisionItem. The generated SQL code
shows also why: ...t0.JDOCLASSX IN ('tolina.data.Account',
'tolina.data.AccountRevisionItem')...
Kodo tries when fetching a set of Accounts also all subclasses of
Accounts! How can i say: this set contains only accounts and no fetching
of subclasses is needed!
I would be grateful for any help!
PS: I noticed that kodo deletes all elements of a list when inserting a
new element, i remember there a several faqs around this. Can you anyway
give a work-a-round?

Answer 1

Hi Patrick,
i found my mistake: in the copy() method of AccountRevisionItem which
copies the attributes of the Accountobject, i copied the Usecase reference
too.
So when all Accounts of a usecase will be fetched the copied
AccountRevisionItem matched too, because of the fk of the usecase :[
I was wondering why i get the RevisionItem although i never added it to
the list of accounts in the usecase - now i know :)
Your strategy to fetch all objects of the desired class AND subclass when
traverse a relation is of course right. But can i control it like in the
case from extents?
Your proposal to use sets instead of lists is not really the solution,
because sets have no order of the containing elements!
I have to life with sets because deleting all elements when inserting a
new one is not really performant...
Thanks anyway for your quick help!
I promise this was not the last time, because we will use kodo in a
prototyp-project - when it will be a success we will buy kodo.
Patrick Linskey wrote:
How are you getting the extent? If you do:
Extent e = pm.getExtent (Account.class, true);
you will get all subclasses. If you do:
Extent e = pm.getExtent (Account.class, false);
then you will get an extent of just Account objects -- no subclasses.
Now, if you're traversing a relation, we will always return all objects
in that relation, regardless of subclass. So, if you have a one-many
relation from UseCase to Account, we'll return all objects in that
relation. However, that relation should only contain data that you put
into it, so if you never put anything but Account objects into it, then
you should not see anything else come out.
Regarding your collection question -- if you declare the field as a
java.util.Set, modifications of the collection will be much more efficient.
-Patrick
Claudius Gr__ver wrote:
Hello again,
there seems to be a problem with one-to-many relations involving
inheritence...
Let me describe the setting:
class AbstractUsecase {
// element-type=Account
Set accounts;
class Usecase extends AbstractUsecase {
class Account extends AbstractAccount implements InstanceCallbacks {
// element-type=AccountRevisionItem
List revisions;
AbstractUsecase uc;
class AccountRevisionItem extends Account {
long time;
User user;
As you can see a simple model (in uml it looks much more better).
In the method jdoPreStore() of Account an object of type
AccountRevisionItem will added to revisions. This new object holds the
data of Account, the User and timestamp of the modifing. So we are able to
trace every change on the date (user, date). Metadata looks like this:
When i get a extent of all Accounts i can fetch all revisions of each
Account.
Now the problem: When i fetch all accounts of a usecase there are objects
included of type Account AND AccountRevisionItem. The generated SQL code
shows also why: ...t0.JDOCLASSX IN ('tolina.data.Account',
'tolina.data.AccountRevisionItem')...
Kodo tries when fetching a set of Accounts also all subclasses of
Accounts! How can i say: this set contains only accounts and no fetching
of subclasses is needed!
I would be grateful for any help!
PS: I noticed that kodo deletes all elements of a list when inserting a
new element, i remember there a several faqs around this. Can you anyway
give a work-a-round?
Patrick Linskey [email protected]
SolarMetric Inc. http://www.solarmetric.com