Makumba list sorting by a subset - subquery

Question

Let's say we have the following Student.mdd:
person=not null ptr general.Person
interests=set general.ActivityField
education=set
education->university=not null ptr general.University
education->graduation=date
I would like to list all students who are interested in painting (where="interests.name='painting'") and sort the list by the latest graduation date.
If I simply use orderBy="student.education.graduation desc", only the first created education will be used for sorting. For example:
Student1 went to University A and graduated in June 2009
Student2 went to University B and graduated in June 2010
Student3 went to University C and graduated in May 2010 and then went to University D and graduated in May 2011
So I expect to get them sorted like this:
Student3 (May 2011)
Student2 (June 2010)
Student1 (June 2009)
But I get this:
Student2 (June 2010)
Student3 (May 2010)
Student1 (June 2009)
Is there a way to force makumba to go through all "education" items for each student, sort them by "graduation" and use the latest date to sort the student?

Answer 1

You most probably need to solve this with a subquery. Let me outline one approach here (in pure OQL now, needs to be adapted for the JSP taglib)
First for comparison, first select a limited number of people
SELECT p, p.name, e.graduation from Student s, s.person p, s.education e
WHERE p.name='name1' or p.name='name2'
ORDER BY p.name, e.graduation DESC
This obviously gives you duplicates if people have more than one study; if you want to group by a person to reduce duplicates, you can't select the latest date though. that is what you implicitly do in your example, I think.
So, you need to limit somehow to select only the education with the latest graduation date for each student, and then sort on that list.
This can be achieved for example like that:
SELECT p, p.name, e.graduation from Student s, s.person p, s.education e
WHERE (p.name='name1' or p.name='name2')
AND e.graduation in
(SELECT max(e2.graduation) from Student s2, s2.education e2 where s=s2)
ORDER BY e.graduation DESC
That selects the latest graduation date for a student in the subquery, and then in the main query selects only that education that has that highest graduation day.
Given, if you have a Student with two studies ending at the same date, you might get duplicates from that. So adding a
'group by s'
might be needed.