Java - Criteria API - Unidirectional OneToMany

Question

I'm trying to get the license number from the person from the process table through criteria API. Basically from Process -> Person -> License

However, Person table has a unidirectional OneToMany relationship with table License.

So I have the following entities:

@Entity
@Table(name = "Process")
public class Process {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    
    @OneToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "person_id")
    Person person;
}

@Entity
@Table(name = "Person")
public class Person {
    @Id
    @Column(name = "id",columnDefinition="INTEGER")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Integer id;
    
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "person")
    List<License> licenses;
}

@Entity
@Table(name = "License")
public class License {
    @Id
    @Column(name = "id",columnDefinition="INTEGER")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Integer id;
    
    @JoinColumn(name = "person_id")
    @ManyToOne(fetch = FetchType.LAZY)
    Person person;
    
    String licenseNumber;
}

In a native query, the result I would want to accomplish would be:

  select lic.license_number, * from Process process
  left join Person p on process.person_id = p.id
  left join License lic on lic.person_id = p.id;

I have tried with Join:

final CriteriaBuilder builder = entityManager.getCriteriaBuilder();
final CriteriaQuery<Process> criteria = builder.createQuery(Process.class);
final Root<Process> rootSelect = criteria.from(Process.class);

//I don't really know how to join rootSelect (Process table), with License table... having the table Person in middle..
Join<Process, Person> personJoin = rootSelect.join("person");
Join<License, Person> licenseJoin = rootSelect.join("person");

and also considered using subquery:

final CriteriaBuilder builder = entityManager.getCriteriaBuilder();
final CriteriaQuery<Process> criteria = builder.createQuery(Process.class);
final Root<Process> rootSelect = criteria.from(Process.class);

Subquery sub = criteria.subquery(String.class);
Root subRoot = sub.from(License.class);
//How to select just the field 'license_number' below?
sub.select(subRoot);
sub.where(builder.equal(rootSelect.get("person").get("id"), subRoot.get("person")));

I will need the license_number for a filter (where) at the end.

What would be the best way to do such a filter, considering my root table is Process?

Answer 1

Strictly speaking, you don't have to join the Process tables with license, you have to join Process with Person and Person with License as follows:

Join<Process, Person> personJoin = rootSelect.join("person",JoinType.INNER);
Join<Person, License> licenseJoin = personJoin.join("licenses",JoinType.INNER);

It is recommended to use metamodels and entities, with them the Join would be as follows (the metamodel is represented by EntityName_):

Join<Process, Person> personJoin = rootSelect.join(Process_.person,JoinType.INNER);
Join<Person, License> licenseJoin = personJoin.join(Person_.licences,JoinType.INNER);

For me the clearest benefit is that you can autocomplete the properties of the metamodel, putting a string increases the chances that we are wrong.

We add the filter condition

cq.where(cb.equal(licenceseJoin.get("licenseNumber"),LICENSE_NUMBER));
//cq.where(cb.equal(licenceseJoin.get(License_.licenseNumber),LICENSE_NUMBER));