用于一对多关系的 Spring Data JPA Select

Question

I have two entities

Box(id, srNo, boxNumber, basePrice, baseAmount)

Lot(id, lotNumber, size, description, pcs, weight, box_id)

One box has many lots
Now, i want to find out box with lot details(boxId, srNo, boxNumber, basePrice, lotId, lotNumber, size)
in a programmatic way(Criteria or queryDSL) with the use of only a single select query

Answer 1

With Criteria API:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<YourPojo> cq = cb.createQuery(YourPojo.class);

Root<Box> rootBox = cq.from(Box.class);
Join<Box,Lot> joinLot = rootBox.join(Box_.lots,JoinType.LEFT);

List<Predicate> predicates = new ArrayList<>();
predicates.add(cb.equal(joinLot.get(Lot_.id), ID_LOT))
cq.where(predicates.toArray(new Predicate[predicates.size()]));

cq.multiselect(
    rootBox.get(Box_.id),
    rootBox.get(Box_.srNo),
    rootBox.get(Box_.boxNumber),
    rootBox.get(Box_.basePrice),
    rootBox.get(Box_.baseAmount),
    joinLot.get(Let_.id),
    joinLot.get(Let_.lotNumber),
    joinLot.get(Let_.size),
    joinLot.get(Let_.description),
    joinLot.get(Let_.pcs),
    joinLot.get(Let_.weight)
);

List<YourPojo> result = entityManager.createQuery(cq).getResultList();

and the following constructor in your YourPojo.java:

public YourPojo(Long id, String srNo, Integer boxNumber, Long basePrice, Long 
    baseAmount, Long idLot, Integer lotNumber, Long size, String description, 
    Integer pcs, Long weight){
     ....
}

In this way you will get an object for each Box-Lot entry.

Another way to do it, for me less optimal is the following: CriteriaBuilder cb = entityManager.getCriteriaBuilder(); CriteriaQuery cq = cb.createQuery(Box.class);

Root<Box> rootBox = cq.from(Box.class);
Join<Box,Lot> joinLot = (Join<Box,Lot>)rootBox.fetch(Box_.lots,JoinType.LEFT);

List<Predicate> predicates = new ArrayList<>();
predicates.add(cb.equal(joinLot.get(Lot_.id), ID_LOT))
cq.where(predicates.toArray(new Predicate[predicates.size()]));

cq.select(rootBox);

List<Box> result = entityManager.createQuery(cq).getResultList();

In this way you will obtain a list of Box entities, with the parameter Lots (List or Lot set) fed.

I prefer the first mode since even if you have N inputs to deal with the streams and lambdas of Java 8 it is very simple, and in case of being an API it is a security flaw to expose entities, in this way you would work with flat objects.