Spring pageable with specification add more LEFT JOIN on order by joined column - spring

Question

Can anybody explain to me why Spring adds LEFT JOIN checking on pageable Order using its specification feature?
Here is the case:
I want to use Spring specification to generate dynamic query for filtering data and pageable to limit the data. The query is between UserEntity and UserGroupEntity, and I want to always fetch the UserGroupEntity on every query, therefore I add a checking block for COUNT and data query. If the query is a query COUNT then I use the Join object, otherwise I use the Fetch object.
public static Specification<UserEntity> filtered(final String searchTerm) {
return new Specification<UserEntity>() {
#SuppressWarnings("unchecked")
#Override
public Predicate toPredicate(Root<UserEntity> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
Join<UserEntity, UserGroupEntity> joinUserGroup = null;
if (QueryHelper.isQueryCount(query)) {
joinUserGroup = root.join("userGroup");
} else {
Fetch<UserEntity, UserGroupEntity> fetchUserGroup = root.fetch("userGroup");
joinUserGroup = (Join<UserEntity, UserGroupEntity>) fetchUserGroup;
}
if (searchTerm != null) {
List<Predicate> predicates = new ArrayList<Predicate>();
predicates.add(cb.like(root.get("lowerUsername").as(String.class), "%" + searchTerm.toLowerCase() + "%"));
predicates.add(cb.like(root.get("lowerEmail").as(String.class), "%" + searchTerm.toLowerCase() + "%"));
predicates.add(cb.like(joinUserGroup.get("lowerName").as(String.class), "%" + searchTerm.toLowerCase() + "%"));
cb.or(predicates.toArray(new Predicate[predicates.size()]));
}
return null;
}
};
}
This is how I call the specification on UserService:
Order order = new Order(Direction.ASC, "lowerEmail");
Page<UserEntity> pages = userRepository.findAll(filtered(searchTerm), new PageRequest(page, size, new Sort(order)));
The query is fine when I add an ordering using UserEntity column. Here is the generated HQL:
select generatedAlias0 from com.example.entity.UserEntity as generatedAlias0
inner join fetch generatedAlias0.userGroup as generatedAlias1
order by generatedAlias0.lowerEmail asc
But the query becomes strange (because I don't know why) when I use the the column from joined entity for ordering which is userGroup.lowerName. By using that column, Spring adds more LEFT JOIN in my query and makes it like this:
select generatedAlias0 from com.example.entity.UserEntity as generatedAlias0
left join generatedAlias0.userGroup as generatedAlias1
inner join fetch generatedAlias0.userGroup as generatedAlias2
order by generatedAlias1.lowerName asc
As I come across the spring-data-JPA code on Github, I found that the translation from specification to criteria is done in method getQuery(). This method calls other method toOrders() from class QueryUtils to apply the sort order. Method toOrders() finally calls method getOrCreateJoin() or isAlreadyFetched() which will check the the previous join attribute (the one I made in the specification) which is userGroup, but because the join type is not LEFT JOIN then Spring adds more join using LEFT JOIN in my query.
private static Join<?, ?> getOrCreateJoin(From<?, ?> from, String attribute) {
for (Join<?, ?> join : from.getJoins()) {
boolean sameName = join.getAttribute().getName().equals(attribute);
if (sameName && join.getJoinType().equals(JoinType.LEFT)) {
return join;
}
}
return from.join(attribute, JoinType.LEFT);
}
That's what I got from my search on the spring-data-jpa code, CMIIW. What is the purpose of the JoinType.LEFT actually I still don't understand. Your explanation will be very helpful for me (and us).
Now I think that I will use custom repository to generate dynamic query using JPQL until I understand the reason for that generated query with additional LEFT JOIN using specification and pageable.

Answer 1

I had the same issue, and found the solution and answer in https://stackoverflow.com/a/43965633/7280115. You may check if it also works for you