How to join two unrelated entities using JPA and Hibernate

Question

I have two tables - one containing Address and another containing Photographs. The only common field between them is the PersonID. These were mapped to two POJO Classes Add

Answer 1

As I explained in this article, you have two options:

1. Since Hibernate 5.1, you can use ad-hoc joins for unrelated entities.

   Tuple postViewCount = entityManager.createQuery(
       "select p as post, count(pv) as page_views " +
       "from Post p " +
       "left join PageView pv on p.slug = pv.slug " +
       "where p.title = :title " +
       "group by p", Tuple.class)
   .setParameter("title", "High-Performance Java Persistence")
   .getSingleResult();
   

2. Prior to Hibernate 5.1, you could only use theta-style joins. However, a theta-style join is equivalent to an equijoin, hence you can only emulate INNER JOINs not OUTER JOINs.

   List postViewCount = entityManager.createQuery(
       "select p as post, count(pv) as page_views " +
       "from Post p, PageView pv " +
       "where p.title = :title and " +
       "      ( p.slug = pv.slug ) " +
       "group by p", Tuple.class)
   .setParameter("title", "Presentations")
   .getResultList();
   

For more details, check out this article.