Criteria query for unidirectional one-to-many relationship

Question

So, I have the following entities:

@Entity
public class Supplier {
    @Column(name = "SUPPLIERID")
    private BigInteger supplierId;

    @OneToMany
    @JoinColumn(name = "ID_SUPP", foreignKey = @ForeignKey(name = "fk_POIS_SUPP"))
    private List<POS> posList;

    ...
}

@Entity
public class POS {
    @Column(name = "POSID")
    private BigInteger posId
}

So, POS does not have a reference to Supplier, which means that we have a unidirectional one-to-many relationship. I need to look for a POS by posId and supplierId. That is, find a supplier with the specified supplierId and then find a pos in the supplier's list of pos's that has the specified posId. How do I write a criteria query for this?

I tried using subqueries. My idea was to create a subquery that would fetch all POS's of a Supplier with a given supplierId. Then the main query would search within those POS's for a POS with the given posId.

The problem was I couldn't write a query that would fetch a Suppliers list of POSs. Apparently you can't write a query of type List<POS>:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<POS> outerQuery = cb.createQuery(POS.class);
Root<POS> outerQueryRoot = outerQuery.from(POS.class);

Subquery<POS> subquery = outerQuery.subquery(POS.class);
Root<Supplier> subqueryRoot = subquery.from(Supplier.class);
subquery.where(cb.equal(subqueryRoot.get(Supplier_.supplierId), supplierId));
subquery.select(subqueryRoot.get(Supplier_.posList);

On this last line, I get a compilation error that Expression<POS> does not match Expression<List<POS>>. And I can't change the type of the subquery because Java doesn't allow generic class literals (List<POS>.class).

Any ideas?

Answer 1

I finally found the answer, just use two roots:

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

    Root<POS> posRoot = cq.from(POS.class);
    Root<Supplier> supplierRoot = cq.from(Supplier.class);

    cq.where(cb.and(
                    cb.equal(supplierRoot.get(Supplier_.suppliertId), supplierId),
                    cb.equal(posRoot.get(POS_.posId), posId)));
    cq.select(posRoot);

Answer 2

I found very simple solution without subquery. Start from Suppler, join POS through posList and then 'select' POS.

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<POS> query = cb.createQuery(POS.class);

Root<Supplier> supplierRoot = query.from(Supplier.class);
ListJoin<Supplier, POS> posList = supplierRoot.joinList(Supplier_.posList);
query
    .select(posList)
    .where(
        cb.equal(supplierRoot.get(Supplier_.suppliertId), supplierId),
        cb.equal(posList.get(POS_.posId), posId)
    );

With Hibernate 5.2.11 it generated nice query with two inner joins through N->M table very similar to manually written code ;-). Accepted answer is I guess wrong because it skips "posList" relation. It will select POS objects which are not in relation with specified Supplier.

Answer 3

You can do this with subqueries. SQL equivalent to the jpql "select p from POS p where p.id in (select sp.id from Supplier s join s.posList sp where s.id = :supplierId)"

See JPA2 Criteria-API: select... in (select from where)