java - 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 Address and Photo. I was able to fetch details in these tables by creating criteria and adding restrictions on the fields . How should we write a join on the two tables. Is it possible to get the result as two objects -Address and Photo.

I want to do a left join so that i can get records of persons without photos as well. I have read that this is possible only using hql but Can this be done using criterias as well?

Answer 1

You can easily write HQL query that will return result as two objects using Theta Join (as Adrian noted). Here is an example:

String queryText = "select address, photo from Address address, Photo photo " 
                 + " where address.personID=photo.personId";
List<Object[]> rows = session.createQuery(queryText).list();

for (Object[] row: rows) {
    System.out.println(" ------- ");
    System.out.println("Address object: " + row[0]);
    System.out.println("Photo object: " + row[1]);
}

As you can see query returns list of Object[] arrays that represents each fetched row. First element of this array will contain one obejct and second element - another.

EDIT:

In case of left join I think you need to use native SQL query (not HQL query). Here how you can do this:

String queryText = "select address.*, photo.* from ADDRESS address 
                    left join PHOTO photo on (address.person_id=photo.person_id)";

List<Object[]> rows = sess.createSQLQuery(queryText)
                          .addEntity("address", Address.class)
                          .addEntity("photo", Photo.class)
                          .list();

This should work for your case.