Horizontal mapping base class query

Question

Greetings,
I have a class hierarchy which looks like the following:
abstract class A {
int num;
class B extends A
class C extends A
class D extends A
This hierarchy is horizontally mapped. Say I want to run the following query. Give me the 10 objects of type A (either B,C or D practically) with the highest num. I would run the following query:
Query query = pm.newQuery("select from A.class range 0, 10");
query.setOrdering("num descending");
I would expect this query to return a maximum of 10 results. It in fact returns up to 30 results, i.e. 10 per subclass type. What is wrong in the structure of the query or my classes? Is this expected behaviour and does anyone know how I may remedy this situation?
Thanks in advance,
Adam.

Answer 1

David-
Did I understand that correctly?You are correct.
If so, why does this limitation exist?It is because a proper relation cannot exist in the database if the
target table is unknown (which it is for horizontally mapped classes),
and joins would become tremendously complex or, in some cases, simply
impossible to accomplish with a single SQL statement.
We are considering adding some support for querying across horizontal
relations that are using non-composite primary keys, but it is currently
not possible.
In article <d3eig3$ssb$[email protected]>, David Ezzio wrote:
Hi guys,
Suppose that we have the following inheritance structure:
Class: LegalEntity
Field: LegalEntity relatedTo
Field: String name
Class: Person extends LegalEntity
Field: String ssn
Class: Corporation extends LegalEntity
Field: String location
Class: CollegeStudent extends Person
Field: String school
Class: Employee extends Person
Field: Corporation company
If a flat mapping is used throughout, then we end up with one table
(LEGALENTITY) with the columns: relatedTo, name, ssn, location, school,
and company, plus the Kodo created jdo columns.
If we wanted to create several tables, we could map LegalEntity and
Person horizontally, making the base tables COLLEGESTUDENT, EMPLOYEE,
and CORPORATION.
If I understand section 7.6.4.3 correctly, the value stored in the
relatedto column will be a stringified object identity, and Kodo does
does not support queries across this relation. Thus, the query find all
CollegeStudent objects that satisfies the filter "relatedTo.name ==
\"Sally\"" is not supported.
Did I understand that correctly?
If so, why does this limitation exist?
David
Marc Prud'hommeaux
SolarMetric Inc.