temp2.java

@Getter
@NoArgsConstructor
@Entity
public class User {

    @OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
    private List<PartyUser> parties = new ArrayList<>();
}


@Getter
@NoArgsConstructor
@Entity
public class Party {

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "leader_id")
    private User leader;

    @OneToMany(mappedBy = "party", fetch = FetchType.LAZY)
    private List<PartyUser> users = new ArrayList<>();
  
}


@NoArgsConstructor
@Getter
@Entity
@Table(
        name="party_user",
        uniqueConstraints={
                @UniqueConstraint(
                        columnNames={"user_id","party_id"}
                )
        }
)
public class PartyUser extends BaseTimeEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne
    @JoinColumn(name = "party_id")
    private Party party;

    @Builder
    public PartyUser(User user, Party party) {
        this.user = user;
        this.party = party;
    }
}

// test
@SpringBootTest
@Transactional
class PartyServiceTest {
    
    @Test
    void 파티장은_강퇴를_할_수_있다() {
        // Given: 내가 바로 파티장. you 는 파티원
        User me = userRepository.save(
                User.builder()
                        .nickName("test")
                        .kakaoId("test-kakao-id")
                        .build()
        );

        User you = userRepository.save(
                User.builder()
                        .nickName("test2")
                        .kakaoId("test-kakao-id2")
                        .build()
        );
        Party party = partyRepository.save(Party.builder().leader(me).build());
        partyUserRepository.save(PartyUser.builder().user(you).party(party).build());

        // When: 파티장이 강퇴를 한다. (party user 삭제)
        partyUserRepository.deleteByPartyIdAndUserId(party.getId(), you.getId());

        // Then: 파티에서 퇴출 완료!
        Party resultParty = partyRepository.getById(party.getId());
        assertThat(resultParty.getUsers().stream().map(PartyUser::getUser).anyMatch(u -> u.getId().equals(you.getId()))).isFalse();
        // 여기서 assert 가 실패합니다! isTrue 가 나옵니다~
    }
}