Question

There are two tables, report and reportcomment. Every reportcomment is assign (via foreign key) to a report, therefore a report has zero to many reportcomments.
If I need a list, that gives me all reports with the respective number of comments of every report, I would do the following (with SQL):

SELECT r.*, c.cnt
FROM report r
LEFT JOIN (
    SELECT ir.id AS report_id, COUNT(ic.id) AS cnt
    FROM report ir
    INNER JOIN reportcomment ic ON ir.id = ic.report_id
    GROUP BY ir.id
) c ON c.report_id = r.id

I would like to retrieve such a list with JPA/Hibernate and store the c.cnt somewhere in my Report entity object.
How could that be achieved?

Answer 1

I think the simpliest way would be to create a transient field in Report and convert tuples returned by the appropriate query manully, something like this:

List<Object[]> tuples = em.createQuery(
    "SELECT r, COUNT(c) " +
    "FROM ReportComment c RIGHT JOIN c.report r" +
    "GROUP BY r.id, ...").getResultList();

List<Report> reports = new ArrayList<Report>();
for (Object[] tuple: tuples) {
    Report r = (Report) tuple[0];
    r.setCommentCount((Long) tuple[1]);
    reports.add(r);
}