How with QueryDSL get the map of ids of one entity and the numbers of associated entities

Question

How to get the map of ids of one entity (key) and the numbers of associated entities (value). Without transfer entities. Something like this:

from(qE1)
   .join(qE2).on(qE2.qE1_id.eq(qE1.id))
   .where(qE1_exp)
   .transform(GroupBy.groupBy(qE2.id).as(qE1.count())) // ?

QueryDSL-4.2.1.

Answer 1

I am not an expert with QueryDSL but I believe something like this or similar using JPAQueryFactory could do it.

Map<Long, Long> entityMap = new JPAQueryFactory(entityManager)
.select(qE2.qE1.id, qE2.count())
.from(qE2)
.join(qE2.qE1)
.where(qE1_exp)
.groupBy(qE2.qE1.id)
.fetch()
.stream()
.collect(Collectors.toMap(
    tuple -> tuple.get(qE2.qE1.id),
    tuple -> tuple.get(qE2.count())
));