출처

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엔티티

@Entity
@Getter
@Setter
@NoArgsConstructor(access = AccessLevel.PROTECTED)
@ToString(of={"id","username","age"})
public class Member {
    @Id @GeneratedValue
    @Column(name="member_id")
    private Long id;
    private String username;
    private int age;

    @ManyToOne(fetch= FetchType.LAZY)
    @JoinColumn(name="team_id")
    private Team team;

    public Member(String username, int age){
        this(username,age,null);
    }
    public Member(String username, int age, Team team){
        this.username = username;
        this.age = age;
        if (team!=null){
            changeTeam(team);
        }
    }

    public void changeTeam(Team team) {
        this.team = team;
        team.getMembers().add(this);
    }

}

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@Entity
@Getter
@Setter
@NoArgsConstructor(access = AccessLevel.PROTECTED)
@ToString(of={"id","name"})
public class Team {

    @Id @GeneratedValue
    private Long id;
    private String name;

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

     public Team(String name){
         this.name = name;
     }

}

Member와 Team이 양방향 연관관계를 갖는다.
연관관계의 주인은 Member이다.

주의점은 toString을 만들 때 연관관계가 있는 필드를 넣어버리면 무한루프를 타면서 오류가 날 수 있어서 가급적이면 소유한 필드만 toString에 넣고 연관관계 필드는 넣지 않는게 좋다.

테스트

@SpringBootTest
@Transactional
class MemberTest {

    @Autowired
    EntityManager em;

    @Test
    public void testEntity(){
        Team teamA = new Team("teamA");
        Team teamB = new Team("teamB");
        em.persist(teamA);
        em.persist(teamB);

        Member member1 = new Member("member1",10,teamA);
        Member member2 = new Member("member2",20,teamA);

        Member member3 = new Member("member3",30,teamB);
        Member member4 = new Member("member4",40,teamB);
        em.persist(member1);
        em.persist(member2);
        em.persist(member3);
        em.persist(member4);

        // 초기화
        em.flush();
        em.clear();

        List<Member> members = em.createQuery("select m from Member m", Member.class).getResultList();

        for (Member member : members) {
            System.out.println("member = " + member);
            System.out.println("-> member.team = " + member.getTeam());
        }
        
    }


}

em.flush : 영속성 컨텍스트에 있는 엔티티들을 쿼리를 날려 db에 반여한다.
em.clear : 영속성 컨텍스트를 깔끔하게 초기화한다

쿼리를 보기 위해 이렇게 해준다.

db에서 결과를 보고 싶으면 @Commit 애노테이션을 달아주면 된다.