4. 연관관계 매핑

4. 연관관계 매핑

객체지향 설계의 목표는 자율적인 객체들의 협력 공동체를 만드는 것이다. - 조영호(객체지향의 사실과 오해)

객체를 테이블에 맞추어 모델링(연관관계가 없는 객체)

참조 대신에 외래 키를 그대로 사용한다면?

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Member {
    @Id
    private Long id;

    private Long teamId;

    private String name;

    public Long getId() {
        return id;
    }

    public void setId(final Long id) {
        this.id = id;
    }

    public Long getTeamId() {
        return teamId;
    }

    public void setTeamId(final Long teamId) {
        this.teamId = teamId;
    }

    public String getName() {
        return name;
    }

    public void setName(final String name) {
        this.name = name;
    }
}

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Team {
    @Id
    private Long id;

    private String name;

    public Long getId() {
        return id;
    }

    public void setId(final Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(final String name) {
        this.name = name;
    }
}

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class MemberTest {

    @DisplayName("테이블에 맞춘 방식")
    @Test
    void SaveAndFindForTable() {
        final EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
        final EntityManager em = emf.createEntityManager();
        final EntityTransaction transaction = em.getTransaction();

        transaction.begin();

        try {
            final Team team = new Team();
            team.setId(1L);
            team.setName("Team A");
            em.persist(team);
            final Member member = new Member();
            member.setId(1L);
            member.setTeamId(1L);
            member.setName("Member A");
            em.persist(member);

            em.flush();
            em.clear();

            final Member findMember = em.find(Member.class, member.getId());

            assertAll(
                    () -> assertThat(findMember.getId()).isEqualTo(member.getId()),
                    () -> assertThat(findMember.getTeamId()).isEqualTo(member.getTeamId()),
                    () -> assertThat(findMember.getName()).isEqualTo(member.getName())
            );

            transaction.commit();
        } catch (final Exception e) {
            transaction.rollback();
        } finally {
            em.close();
        }

        emf.close();
    }

}

식별자로 다시 조회, 객체 지향적인 방법이 아니다.

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class MemberTest {

    @DisplayName("팀을 찾는 테스트")
    @Test
    void findTeam() {
        final EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
        final EntityManager em = emf.createEntityManager();
        final EntityTransaction transaction = em.getTransaction();

        transaction.begin();

        try {
            final Team team = new Team();
            team.setId(1L);
            team.setName("Team A");
            em.persist(team);
            final Member member = new Member();
            member.setId(1L);
            member.setTeamId(1L);
            member.setName("Member A");
            em.persist(member);

            em.flush();
            em.clear();

            final Member findMember = em.find(Member.class, member.getId());
            final Team findTeam = em.find(Team.class, findMember.getTeamId());

            assertAll(
                    () -> assertThat(findTeam.getName()).isEqualTo(team.getName())
            );

            transaction.commit();
        } catch (final Exception e) {
            transaction.rollback();
        } finally {
            em.close();
        }

        emf.close();
    }

}

객체를 테이블에 맞추어 데이터 중심으로 모델링하면, 협력 관계를 만들 수 없다.

• 테이블은 외래 키로 조인을 사용해서 연관된 테이블을 찾는다.
• 객체는 참조를 사용해서 연관된 객체를 찾는다.
• 테이블과 객체 사이에는 이런 큰 간격이 있다.

단방향 매핑

객체 지향 모델링 (객체 연관관계 사용)

• 여기서 객체에서는 Member가 Team을 알고 있지만 Team이 Member를 알고 있지 않다.
• 그러나 DB는 Join을 통해 둘다 가능하다.

ORM 매핑

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;

@Entity
public class Member {
    @Id
    private Long id;

    @ManyToOne
    @JoinColumn(name = "team_id")
    private Team team;

    private String name;

    public Member() {
    }

    public Member(final Long id, final Team team, final String name) {
        this.id = id;
        this.team = team;
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public Team getTeam() {
        return team;
    }

    public String getName() {
        return name;
    }
}

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class MemberTest {

    @DisplayName("객체에 맞춘 방식")
    @Test
    void SaveAndFindForObj() {
        final EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
        final EntityManager em = emf.createEntityManager();
        final EntityTransaction transaction = em.getTransaction();

        transaction.begin();

        try {
            final Team team = new Team(1L, "Team A");
            em.persist(team);
            final Member member = new Member(1L, team, "Member A");
            em.persist(member);

            em.flush();
            em.clear();

            final Member findMember = em.find(Member.class, member.getId());

            assertAll(
                    () -> assertThat(findMember.getId()).isEqualTo(member.getId()),
                    () -> assertThat(findMember.getTeam().getId()).isEqualTo(member.getTeam().getId()),
                    () -> assertThat(findMember.getName()).isEqualTo(member.getName())
            );

            transaction.commit();
        } catch (final Exception e) {
            transaction.rollback();
        } finally {
            em.close();
        }

        emf.close();
    }
}

FetchType

JPA 페치(Fetch) 전략 - 즉시 로딩(EAGER)과 지연 로딩(LAZY)

연관관계 수정

• setter로 손쉽게 변경이 가능하다. (따로 update 쿼리를 위해 무언가를 할 필요가 없다.)

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class MemberTest {

    @DisplayName("업데이트 테스트")
    @Test
    void updateTest() {
        final EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
        final EntityManager em = emf.createEntityManager();
        final EntityTransaction transaction = em.getTransaction();

        transaction.begin();

        try {
            final Team team = new Team(1L, "Team A");
            em.persist(team);
            final Member memberA = new Member(1L, team, "Member A");
            em.persist(memberA);

            em.flush();
            em.clear();

            final Member findMember = em.find(Member.class, memberA.getId());
            findMember.setName("Member B");

            em.flush();
            em.clear();

            final Member findMember2 = em.find(Member.class, memberA.getId());

            assertThat(findMember2.getName()).isEqualTo("Member B");

            transaction.commit();
        } catch (final Exception e) {
            transaction.rollback();
        } finally {
            em.close();
        }

        emf.close();

    }
}

• 아래와 같이 update 쿼리가 날라가는걸 확인할 수 있습니다.

update
    Member
set name=?,
    team_id=?
where id = ?

양방향 매핑

@Entity
public class Team {
    @Id
    private Long id;

    private String name;

    @OneToMany(mappedBy = "team")
    private final List<Member> members = new ArrayList<>();

    public Team() {
    }

    public Team(final Long id, final String name) {
        this.id = id;
        this.name = name;
    }

    public List<Member> getMembers() {
        return members;
    }

    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class MemberTest {
    @DisplayName("양방향 테스트")
    @Test
    void bidirectionalTest() {
        final EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
        final EntityManager em = emf.createEntityManager();
        final EntityTransaction transaction = em.getTransaction();

        transaction.begin();

        try {
            final Team team = new Team(1L, "Team A");
            em.persist(team);
            final Member memberA = new Member(1L, team, "Member A");
            em.persist(memberA);
            final Member memberB = new Member(1L, team, "Member B");
            em.persist(memberB);

            em.flush();
            em.clear();

            final Team findTeam = em.find(Team.class, team.getId());

            assertThat(findTeam.getMembers()).hasSize(2);

            transaction.commit();
        } catch (final Exception e) {
            transaction.rollback();
        } finally {
            em.close();
        }

        emf.close();
    }
}

출처

[토크ON세미나] JPA 프로그래밍 기본기 다지기 4강 - 연관관계 매핑 | T아카데미