Subquery - JPA - Include Subquery as part of the main selection - sql

Question

I have an issue to add the result of a subquery as part of the selection.
This is how my data model looks like:
#Entity
#Table(name = "flow")
public class Flow {
#Id
Long id;
#OneToMany(mappedBy="flow", fetch = FetchType.LAZY)
Set<Subscription> subscribers;
... other internal fields
//extra fields that come from other tables
#Transient //as it is not part of the model sometimes must be null
Long numberOfActiveSubscribers;
}
#Entity
#Table(name = "subscriptions")
public class Subscription {
#Id
Long id;
#ManyToOne
#JoinColumn(name = "idflow")
Flow flow;
#OneToMany(name = "user")
User user;
#Column(name = "active")
boolean active;
}
I need to implement a query using jpa specifications that will include in its definition the number of user that have active subscriptions to the flow. So that I will be able to use pagination over that, including sort by this field that is not part of the original flow table. The SQL query I came out with look like this:
SELECT f.*,
(SELECT count(sf.id)
FROM subscription sf
WHERE sf.active = true
AND sf.idf = f.id) as numberofactivesubscribers,
FROM flow f;
I would like to us it in a findAll method like so:
this.repository.findAll(new Specification<Flow>() {
#Override
public Predicate toPredicate(Root<Flow> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder) {
Subquery<Long> sq = query.subquery(Long.class);
Root<Subscription> subsRoot = sq.from(Subscription.class);
sq.select(cb.count(subsRoot.get("id")))
.where(cb.equal(subsRoot.get("flow"), flowRoot),
cb.equal(subsRoot.get("active"), true));
//I DONT KNOW HOW TO INCLUDE THIS AS A PART OF THE MAIN SELECT OF THIS QUERY
}
}, pagination);
But as you can see I don't know how to include the subquery as part of the select method as I did in my SQL query.