Hi, Chris
An exam paper can have 100+ questions, so I use pagination.
ChoiceQuestion does not have any relationship defined. Adding a
ManyToOne relationship to it should work, but there is a possibility
that we change it to a ManyToMany relationship.
So I think your last suggestion fits us the most.
Thanks for your help on this and my previous questions.
On 11/8/2011 11:24 PM, Christopher Delahunt wrote:
Hello
Warren,
Can you describe the relationships you have from ChoiceQuestions?
Is there a reason why you are using a query with pagination to
read in the questions instead of just accessing the relationship
on the particular Exam?
Some options: If you do not need to use the relationships from the
ChoiceQuestion, you can mark them as lazy so they are only queried
when needed. A second option is to not map the relationships from
the ChoiceQuestion side and instead query for them when needed (as
you are doing with the Exam->Questions already).
A third option is to break up the ChoiceQuestion query into
chunks, or to read in all the ChoiceQuestion pks and then read in
the full objects a bunch at a time. Ie,
"select questions from ExamPaper p join p.questions questions
where p.id=:paperId and mod(questions.id, 2) = :split"
Or
"select questions.id from ExamPaper p join p.questions questions
where p.id=:paperId" then
"select questions from ChoiceQuestions questions where
questions.id in(:listOfIds)"
This would allow you to use joining or batching on any
relationships in ChoiceQuestion.
Best Regards,
Chris
On 07/11/2011 10:41 AM, Warren Tang wrote:
But fetch join on OneToMany/ManyToMany
relationship has problems with
pagination.
The problem seems to be called N+1 select problem. I searched
hard but
could found a solution.
It's a problem everyone faces, right? How do you solve it?
Regards,
Warren Tang <http://blog.tangcs.com>
On Friday, November 04, 2011 5:43:32 PM, Leon Derks wrote:
Hello.
I think you can do this with a fetch join like:
SELECT pub FROM Publisher pub JOIN pub.magazines mag
Leon
On Nov 4, 2011, at 10:36 AM, Warren Tang wrote:
Hi, everybody
I have an ExamPaper entity which has many ChoiceQuestions.
The
questions are lazy loaded by default (fetch=LAZY).
Now I want to get all the quesitions that belong to a
specific
ExamPaper, so I use the following JPQL:
"select p.questions from ExamPaper p where p.id=:paperId";
The problem is that questions are "lazy" loaded, so there is
one
"SELECT" executed for *every single* question. But I want to
select
them all in one go for performance reasons. What should I
do?
------------------ Code listing
-----------------------------------
@Entity
public class ExamPaper {
public static final String SELECT_QUESTION_BY_PAPER =
"select
p.questions from ExamPaper p where p.id=:paperId";
@Id @GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(fetch = FetchType.LAZY, orphanRemoval = true,
cascade =
{CascadeType.ALL})
List<ChoiceQuestion> questions;
... ...
}
query = em.createQuery(ExamPaper.SELECT_QUESTION_BY_PAPER,
ChoiceQuestion.class);
query.setParameter("paperId", examPaperId);
query.setFirstResult(startPosition);
query.setMaxResults(maxResult);
list = query.getResultList();
--
Regards,
Warren Tang <http://blog.tangcs.com/>
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