What is the best way of returning group-by sql results in Toplink?

Question

I have many-to-many relationship between Employee and Project; so,
a Employee can have many Projects, and a Project can be owned by many Employees.
I have three tables in the database:
Employee(id int, name varchar(32)),
Project(id int, name varchar(32)), and
Employee_Project(employee_id int, project_id int), which is the join-table between Employee and Project.
Now, I want to find out for each employee, how many projects does the employee has.
The sql query that achieves what I want would look like this:
select e.id, count(*) as numProjects
from employee e, employee_project ep
where e.id = ep.employee_id
group by e.id
Just for information, currently I am using a named ReadAllQuery and I write my own sql in
the Workbench rather than using the ExpressionBuilder.
Now, my two questions are :
1. Since there is a "group by e.id" on the query, only e.id can appear in the select clause.
This prevent me from returning the full Employee pojo using ReadAllQuery.
I can change the query to a nested query like this
select e.eid, e.name, emp.cnt as numProjects
from employee e,
(select e_inner.id, count(*) as cnt
from employee e_inner, employee_project ep_inner
where e_inner.id = ep_inner.employee_id
group by e_inner.id) emp
where e.id = emp.id
but, I don't like the complication of having extra join because of the nested query. Is there a
better way of doing something like this?
2. The second question is what is the best way of returning the count(*) or the numProjects.
What I did right now is that I have a ReadAllQuery that returns a List<Employee>; then for
each returned Employee pojo, I call a method getNumProjects() to get the count(*) information.
I had an extra column "numProjects" in the Employee table and in the Employee descriptor, and
I set this attribute to be "ReadOnly" on the Workbench; (the value for this dummy "numProjects"
column in the database is always 0). So far this works ok. However, since the numProjects is
transient, I need to set the query to refreshIdentityMapResult() or otherwise the Employee object
in the cache could contain stale numProjects information. What I worry is that refreshIdentityMapResult()
will cause the query to always hit the database and beat the purpose of having a cache. Also, if
there are multiple concurrent queries to the database, I worry that there will be a race condition
of updating this transient "numProjects" attribute. What are the better way of returning this kind
of transient information such as count(*)? Can I have the query to return something like a tuple
containing the Employee pojo and an int for the count(*), rather than just a Employee pojo with the
transient int inside the pojo? Please advise.
I greatly appreciate any help.
Thanks,
Frans

Answer 1

No I don't want to modify the set of attributes after TopLink returns it to me. But I don't
quite understand why this matters?
I understand that I can use ReportQuery to return all the Employee's attributes plus the int count(*)
and then I can iterate through the list of ReportQueryResult to construct the Employee pojo myself.
I was hesitant of doing this because I think there will be a performance cost of not being able to
use lazy fetching. For example, in the case of large result sets and the client only needs a few of them,
if we use the above aproach, we need to iterate through all of them and wastefully create all the Employee
pojos. On the other hand, if we let Toplink directly return a list of Employee pojo, then we can tell
Toplink to use ScrollableCursor and to fetch only the first several rows. Please advise.
Thanks.