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BAEL-8174 - Implementing Unions in Hibernate (#18577)
* initial code * bael-8174 - article ready for review * bael-8174 - review 1: lecturer/researcher * bael-8174 - review 2
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‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/UnionApplication.java

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package com.baeldung.hibernate.union;
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import org.springframework.boot.SpringApplication;
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import org.springframework.boot.autoconfigure.SpringBootApplication;
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@SpringBootApplication
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public class UnionApplication {
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public static void main(String[] args) {
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SpringApplication.run(UnionApplication.class, args);
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}
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/dto/PersonDto.java

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package com.baeldung.hibernate.union.dto;
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import java.util.Objects;
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public class PersonDto {
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private Long id;
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private String name;
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private String role;
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public PersonDto(Long id, String name) {
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this.id = id;
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this.name = name;
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}
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public PersonDto(Long id, String name, String role) {
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this(id, name);
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this.role = role;
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}
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public Long getId() {
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return id;
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}
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public void setId(Long id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public String getRole() {
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return role;
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}
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public void setRole(String role) {
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this.role = role;
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}
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@Override
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public int hashCode() {
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return Objects.hash(id, name);
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}
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@Override
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public boolean equals(Object obj) {
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if (this == obj)
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return true;
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if (obj == null)
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return false;
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if (getClass() != obj.getClass())
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return false;
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PersonDto other = (PersonDto) obj;
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return Objects.equals(id, other.id) && Objects.equals(name, other.name);
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}
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/model/Lecturer.java

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package com.baeldung.hibernate.union.model;
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import jakarta.persistence.Entity;
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import jakarta.persistence.Id;
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@Entity
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public class Lecturer {
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@Id
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private Long id;
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private String name;
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private Integer facultyId;
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public Lecturer() {
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}
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public Lecturer(Long id, String name) {
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this.id = id;
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this.name = name;
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}
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public Long getId() {
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return id;
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}
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public void setId(Long id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public Integer getFacultyId() {
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return facultyId;
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}
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public void setFacultyId(Integer facultyId) {
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this.facultyId = facultyId;
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}
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/model/PersonView.java

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package com.baeldung.hibernate.union.model;
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public interface PersonView {
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Long getId();
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String getName();
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String getRole();
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/model/Researcher.java

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package com.baeldung.hibernate.union.model;
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import jakarta.persistence.Entity;
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import jakarta.persistence.Id;
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@Entity
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public class Researcher {
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@Id
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private Long id;
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private String name;
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private boolean active;
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public Researcher() {
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}
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public Researcher(Long id, String name) {
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this.id = id;
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this.name = name;
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}
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public Long getId() {
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return id;
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}
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public void setId(Long id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public boolean isActive() {
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return active;
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}
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public void setActive(boolean active) {
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this.active = active;
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}
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/repository/LecturerRepository.java

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package com.baeldung.hibernate.union.repository;
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import java.util.List;
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import org.springframework.data.jpa.repository.JpaRepository;
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import org.springframework.data.jpa.repository.Query;
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import org.springframework.stereotype.Repository;
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import com.baeldung.hibernate.union.model.Lecturer;
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import com.baeldung.hibernate.union.model.PersonView;
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@Repository
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public interface LecturerRepository extends JpaRepository<Lecturer, Long> {
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@Query(value = "select e.id, e.name, e.role from person_view e", nativeQuery = true)
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List<PersonView> findPersonView();
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/repository/ResearcherRepository.java

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package com.baeldung.hibernate.union.repository;
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import org.springframework.data.jpa.repository.JpaRepository;
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import org.springframework.stereotype.Repository;
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import com.baeldung.hibernate.union.model.Researcher;
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@Repository
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public interface ResearcherRepository extends JpaRepository<Researcher, Long> {
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}

‎persistence-modules/hibernate-jpa-3/src/main/java/com/baeldung/hibernate/union/service/UnionService.java

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package com.baeldung.hibernate.union.service;
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import java.util.List;
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import java.util.Set;
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import java.util.stream.Collectors;
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import java.util.stream.Stream;
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import org.hibernate.Session;
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import org.springframework.stereotype.Service;
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import com.baeldung.hibernate.union.dto.PersonDto;
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import com.baeldung.hibernate.union.model.Lecturer;
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import com.baeldung.hibernate.union.model.Researcher;
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import com.baeldung.hibernate.union.repository.LecturerRepository;
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import com.baeldung.hibernate.union.repository.ResearcherRepository;
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import jakarta.persistence.EntityManager;
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import jakarta.persistence.PersistenceContext;
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import jakarta.persistence.criteria.CriteriaQuery;
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import jakarta.persistence.criteria.Root;
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@Service
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public class UnionService {
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@PersistenceContext
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private EntityManager em;
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private LecturerRepository lecturerRepository;
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private ResearcherRepository researcherRepository;
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public UnionService(LecturerRepository lecturerRepository, ResearcherRepository researcherRepository) {
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this.lecturerRepository = lecturerRepository;
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this.researcherRepository = researcherRepository;
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}
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public List<PersonDto> fetch() {
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return em.createQuery("""
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select new PersonDto(l.id, l.name) from Lecturer l
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union
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select new PersonDto(r.id, r.name) from Researcher r
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""", PersonDto.class)
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.getResultList();
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}
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public List<PersonDto> fetchAll() {
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return em.createQuery("""
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select new PersonDto(l.id, l.name) from Lecturer l
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union all
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select new PersonDto(r.id, r.name) from Researcher r
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""", PersonDto.class)
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.getResultList();
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}
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public List<PersonDto> fetchWithDiscriminator() {
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return em.createQuery("""
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select new PersonDto(l.id, l.name, 'LECTURER') from Lecturer l
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union
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select new PersonDto(r.id, r.name, 'RESEARCHER') from Researcher r
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""", PersonDto.class)
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.getResultList();
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}
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public List<PersonDto> fetchManually() {
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List<Lecturer> lecturers = lecturerRepository.findAll();
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List<Researcher> researchers = researcherRepository.findAll();
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return Stream.concat(lecturers.stream()
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.map(l -> new PersonDto(l.getId(), l.getName(), "LECTURER")),
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researchers.stream()
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.map(r -> new PersonDto(r.getId(), r.getName(), "RESEARCHER")))
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.toList();
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}
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public Set<PersonDto> fetchSetManually() {
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List<Lecturer> lecturers = lecturerRepository.findAll();
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List<Researcher> researchers = researcherRepository.findAll();
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return Stream.concat(lecturers.stream()
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.map(l -> new PersonDto(l.getId(), l.getName(), "LECTURER")),
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researchers.stream()
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.map(r -> new PersonDto(r.getId(), r.getName(), "RESEARCHER")))
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.collect(Collectors.toSet());
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}
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@SuppressWarnings("unchecked")
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public List<PersonDto> fetchView() {
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return em.createNativeQuery("select e.id, e.name, e.role from person_view e", PersonDto.class)
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.getResultList();
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}
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public List<PersonDto> fetchWithCriteria() {
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var session = em.unwrap(Session.class);
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var builder = session.getCriteriaBuilder();
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CriteriaQuery<PersonDto> lecturerQuery = builder.createQuery(PersonDto.class);
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Root<Lecturer> lecturer = lecturerQuery.from(Lecturer.class);
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lecturerQuery.select(builder.construct(PersonDto.class, lecturer.get("id"), lecturer.get("name"), builder.literal("LECTURER")));
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CriteriaQuery<PersonDto> researcherQuery = builder.createQuery(PersonDto.class);
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Root<Researcher> researcher = researcherQuery.from(Researcher.class);
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researcherQuery.select(builder.construct(PersonDto.class, researcher.get("id"), researcher.get("name"), builder.literal("RESEARCHER")));
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var unionQuery = builder.unionAll(lecturerQuery, researcherQuery);
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return session.createQuery(unionQuery)
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.getResultList();
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}
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}

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