결국은 JPA를 사용한다.

• fk는 many에 있어야 한다.

@Entity
@Getter
@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("team_id")
	private Team team;

	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);
	}
}

• 연관관계 필드는 ToString 안하는 것이 좋다. 무한 루프에 빠질 수 있다.
• changeTeam 이라는 연관관계 세팅 메소드를 만들어 주었다.

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

	@Id @GeneratedValue
	@Column(name = "team_id")
	private Long id;
	private String name;

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

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

• 양방향 관계에서 관계의 주인을 정해 주었다.

@SpringBootTest
@Transactional
@Rollback(false)
class MemberTest {

	@PersistenceContext
	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();

		// 확인
		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());
		}
	}
}

의도 했던 대로 잘 나온다.