Spring Data JPA: CriteriaQuery to get entities with max value for each unique foreign key - sql

Question

There's an Event class:
#Entity
public class Event {
#Id
private Integer id;
#ManyToOne(cascade = CascadeType.ALL)
private Company company;
#Column
private Long time;
...
}
I want to have an EventFilter class (implementing Specification) which will produce CriteriaQuery to select entities the same way as the following SQL query:
SELECT *
FROM events e1
WHERE e1.time = (
SELECT MAX(time)
FROM events e2
WHERE e1.company_id = c2.company_id
)
Filtered result will contain only events with unique Company and max time value per company.
This is the EventFilter class with what I ended up with:
public class EventFilter implements Specification<Event> {
#Override
public Predicate toPredicate(Root<Event> root, CriteriaQuery<?> q, CriteriaBuilder cb) {
Subquery<Long> subquery = q.subquery(Long.class);
Root<Event> subRoot = subquery.from(Event.class);
subquery.select(cb.max(root.get("time")))
.where(cb.equal(root.get("company"), subRoot.get("company")));
return cb.equal(root.get("time"), subquery);
}
}
When EventRepository#findAll(EventFilter filter) is called, results are not filtered at all. Please help me to implement this logic correctly.

Answer 1

After inspecting SQL statement generated by Hibernate I've found an error: root was used instead of subRoot. The correct method body is:
Subquery<Long> sub = q.subquery(Long.class);
Root<Event> subRoot = sub.from(Event.class);
sub.select(cb.max(subRoot.get("time")))
.where(cb.equal(root.get("company"), subRoot.get("company")));
return cb.equal(root.get("time"), sub);