We Keep Coding

Spring framework data JPA Inner Join on multiple columns

I would like to do the following query using spring jpa. I am able to build the where clause for my statement with Predicate.toPredicate. However, I don't know how to join on more than one column. On a single column it can be done in the repository using #Query and #Param
SELECT a.name, a.column_x, b.column_y
FROM table_a a
INNER JOIN table_b b
ON b.name = a.name
AND b.pk_2 = a.pk_2
AND b.pk_3 = a.pk_3
WHERE ...;
Another question I have is, is an intermediate tableA_tableB association beneficial if I have something like this, oneToMany relations.
Table 1: thing
thing_name
type
tenant
other1
other2
Table 2: thing_sub_prop
prop_name
value
Association table: thing_thing_sub_prop
type
thing_name
tenant
prop_name
value
Or is it better to just have two tables, thing and thing_sub_prop with the primary key columns of thing repeated in thing_sub_prop as a foreign key?

Left join on unrelated tables in Query DSL and JPA

I have two unrelated tables, each one with field email. I need a query which introduces column taken from second table if emails match or will be null if no match is found. In SQL this is easy:
SELECT tableA.id, tableA.email, tableB.name
FROM tableA
LEFT JOIN tableB ON tableA.email=tableB.email
ORDER BY tableB.name
Unfortunately JPA doesn't allow joins over unrelated entities so I converted it to:
SELECT tableA.id, tableA.email,
(SELECT tableB.name FROM tableB WHERE tableB.email=tableA.email) AS aname
FROM tableA
ORDER BY aname
Now, it works as JPA query but we are using Query DSL so off we go to converting it:
JPQLQuery query = new JPAQuery(em);
List<Dto> items=query.from(qTableA)
.list(new QDto(qTableA.id, qTableA.email,
new JPASubQuery().from(qTableB)
.where(qTableB.email.eq(qTableA.email)).unique(qTableB.name)))
It works but now I have no idea how to implement sorting and filtering by field introduced by subquery.
Dto is a POJO used to collect results, QDto is a class autogenerated from Dto.
Question is: how to join two unrelated tables using Query DSL and JPA and avoiding native SQL? Is it possible? Sorting and filtering on fields from tableA and tableB.name is a requirement.

Join on unrelated entities is not covered by latest JPA spec (2.1)
However Hibernate 5.1.0+ and EclipseLink 2.4.0+ support ad hoc joins. http://blog.anthavio.net/2016/03/join-unrelated-entities-in-jpa.html

How can I annotate this complex join in JPA?

Let's say I have the following tables:
Locations
================
LocationKey, LocationName
Employees
================
EmployeeKey, FirstName, LastName
EmployeeLocationXRef
================
EmployeeLocationXRefKey, LocationKey, EmployeeKey
TimeSheets
=================
TimeSheetKey, EmployeeLocationXRefKey, Minutes
OK, so as you can see, in order for me to get all of the TimeSheets for a Location and Employee, I can do the following in SQL
select
*
from TimeSheets ts
join EmployeeLocationXRef ex on (ex.EmployeeLocationXRefKey = ts.EmployeeLocationXRefKey)
join Locations l on (l.LocationKey = ex.LocationKey)
join Employees e on (e.EmployeeKey = ex.EmployeeKey)
where
l.LocationKey = 'xxxx'
and e.EmployeeKey = 'yyyy'
But I have tried every way I can think of to map with (with annotations) in JPA using #JoinTable, etc.
Any ideas how this could be done? Unfortunately, this is a legacy system and the schema cannot be changed.
EDIT
Forgot to mention the EmployeeLocationXRef entity has not been directly mapped. Which could be the real problem. I will see about creating that entity mapping and see if it makes it easier.

I don't see anything particularly difficult or strange about this schema. There should simply be one entity per table and (unless there are unique constraints):
a ManyToOne between EmployeeLocationXRef and Location, using a JoinColumn
a ManyToOne between EmployeeLocationXRef and Employee, using a JoinColumn
a ManyToOne between Timesheet and EmployeeLocationXRef, using a JoinColumn
(these associations can of course be made bidirectional, or be in the other direction if you prefer so).
The JPQL query would simply be
select ts from Timesheet ts
join ts.employeeXRef ex
join ex.location l
join ex.employee e
where
l.locationKey = 'xxxx'
and e.employeeKey = 'yyyy'
which is a straightforward translation of the SQL query.

Hibernate Native SQL Query retrieving multiple entities in join

Referencing with this answer to this correlated thread,
the trick posted by ehrhardt works fine.
But, what I have to do if I have to join with multiple entities? for example:
List<Person> peopleWithBooks = session.createSQLQuery(
"select {p.*}, {b.*}, {m.*} from person p, book b, magazine m where <complicated join>")
.addEntity("p", Person.class)
.addJoin("b", "p.books")
.addJoin("m", "p.magazines")
.addEntity("p", Person.class)
.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
.list();
Hibernate aggregates right the first join, but not the second (magazine entities are not grouped).
There are any tricks or there is a limit to join with only one correlated entity?
And if I have to join with entities that have sub entities? (my goal is to retrieve all selected data with only one custom query)

Inner join using HQL

I am trying to inner join two tables on one column.
From DB side, there's no mapping as it's something I don't want to discuss.
I want to execute HQL query using INNER JOIN and retrieve ROLE objects/results.
Here's my hql so far
session.createQuery("from ROLE as role INNER JOIN INVOLVEMENT as involvement ON role.id = involvement.roleid WHERE involvement.id = X").list();
I see ON is not available on HQL. how do i explicitly tell Hibernate to JOIN on this column only.
I tried below one too
select roleSpec from ROLE as role, INVOLVEMENT as involvement WHERE role.ID = involvement.role_id and involvement.id =27251352
But I am getting ROLE not mapped in exception.

Please check that your ROLE is indeed a mapped entity.
In addition, you don't need to perform "ON" - hibernate knows what is the join column (I know how to define this at JPA ) - so no need to provide it at the statement.
It should be -
session.createQuery("from Role as role INNER JOIN Involvement as involvement WHERE involvement.id = X").list();
I assume you have Role class mapped to ROLE table, and Involvement class mapped to Involement table.
Maybe you used table names by mistake, and this is why you get the "not mapped" error.
Last time I wrote HQL (and not JPA-QL) I used the following link as reference, it provides all the info needed.