다중 엔티티 그래프를 사용할 때 주의해야할 점

FETCH JOIN을 사용할 때 제약사항이 있었다.
ToOne 관계 일때는 여러번 사용이 가능했으나, ToMany 관계는 한번만 사용이 가능하다.

Hospital.java

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.
.

 @OneToMany(mappedBy = "hospitals", cascade = CascadeType.REMOVE, fetch = FetchType.LAZY)
    @OrderBy("id asc")
    private List<Member> members;

Member.java

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.
.
 @JsonIgnore
   @ManyToOne
   @JoinColumn(name = "hospitals_id")
   private Hospital hospitals;

@OneToMany(mappedBy = "members", cascade = CascadeType.REMOVE, fetch = FetchType.LAZY)
   private List<Equipment> equipments;

Equipment.java

@JsonIgnore
@ManyToOne
@JoinColumn(name = "members_id")
private Member members;

Hospital 안에 Member, Member안에 Equipments 이런 구조를 가지고 있다.
여기 있는 모든 데이터를 조회 하려고 Repository에서 JPQL을 작성하였다.

@Query("SELECT DISTINCT h " +
            "FROM Hospital h " +
            "LEFT JOIN FETCH h.members m " +
            "LEFT JOIN FETCH m.equipments e " +
            "ORDER BY h.hospitalName")
List<Team> findAllWithMembersAndEquipments();

실행 하니

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags: 

이런 에러가 발생하였다.

그래서 Fetch Join을 제거하고

@Query("SELECT DISTINCT h " +
            "FROM Hospital h " +
            "LEFT JOIN h.members m " +
            "LEFT JOIN m.equipments e " +
            "ORDER BY h.hospitalName")
List<Team> findAllWithMembersAndEquipments();

이렇게 수정하니

Hibernate: 
    select
        distinct hospital0_.hospital_id as hospital1_1_,
        hospital0_.hospital_address as hospital2_1_,
        hospital0_.hospital_code as hospital3_1_,
        hospital0_.hospital_name as hospital4_1_,
        hospital0_.hospital_number as hospital5_1_ 
    from
        hospital hospital0_ 
    left outer join
        member members1_ 
            on hospital0_.hospital_id=members1_.hospitals_id 
    left outer join
        equipment equipments2_ 
            on members1_.member_id=equipments2_.members_id 
    order by
        hospital0_.hospital_name
Hibernate: 
    select
        members0_.hospitals_id as hospital5_2_1_,
        members0_.member_id as member_i1_2_1_,
        members0_.member_id as member_i1_2_0_,
        members0_.hospitals_id as hospital5_2_0_,
        members0_.member_name as member_n2_2_0_,
        members0_.nickname as nickname3_2_0_,
        members0_.password as password4_2_0_ 
    from
        member members0_ 
    where
        members0_.hospitals_id in (
            ?, ?, ?, ?, ?
        ) 
    order by
        members0_.member_id asc
Hibernate: 
    select
        equipments0_.members_id as members_4_0_1_,
        equipments0_.equipment_id as equipmen1_0_1_,
        equipments0_.equipment_id as equipmen1_0_0_,
        equipments0_.members_id as members_4_0_0_,
        equipments0_.model_name as model_na2_0_0_,
        equipments0_.model_serial_no as model_se3_0_0_ 
    from
        equipment equipments0_ 
    where
        equipments0_.members_id in (
            ?, ?, ?, ?, ?, ?
        )

Hospital, Member, Equipment 세개를 조회할 때 Select 쿼리가 중복되는 N+1 문제가 발생하게 되었다.
해결책은 간단하게, List인 부분을 Set으로 변경한다.
Hospital.java

    @OneToMany(mappedBy = "hospitals", cascade = CascadeType.REMOVE, fetch = FetchType.EAGER)
    @OrderBy("id asc")
    private Set<Member> members;

Member.java

    @OneToMany(mappedBy = "members", cascade = CascadeType.REMOVE, fetch = FetchType.EAGER)
    private Set<Equipment> equipments;

Repository

    @Query("SELECT DISTINCT h " +
            "FROM Hospital h " +
            "LEFT JOIN h.members m " +
            "LEFT JOIN m.equipments e " +
            "ORDER BY h.hospitalName")
    @EntityGraph(attributePaths = {"members", "members.equipments"})
    Set<Hospital> findAllWithMembersAndEquipments();

그리고 EntityGraph로 가져온다.
결과

Hibernate: 
    select
        distinct hospital0_.hospital_id as hospital1_1_0_,
        members1_.member_id as member_i1_2_1_,
        equipments2_.equipment_id as equipmen1_0_2_,
        hospital0_.hospital_address as hospital2_1_0_,
        hospital0_.hospital_code as hospital3_1_0_,
        hospital0_.hospital_name as hospital4_1_0_,
        hospital0_.hospital_number as hospital5_1_0_,
        members1_.hospitals_id as hospital5_2_1_,
        members1_.member_name as member_n2_2_1_,
        members1_.nickname as nickname3_2_1_,
        members1_.password as password4_2_1_,
        members1_.hospitals_id as hospital5_2_0__,
        members1_.member_id as member_i1_2_0__,
        equipments2_.members_id as members_4_0_2_,
        equipments2_.model_name as model_na2_0_2_,
        equipments2_.model_serial_no as model_se3_0_2_,
        equipments2_.members_id as members_4_0_1__,
        equipments2_.equipment_id as equipmen1_0_1__ 
    from
        hospital hospital0_ 
    left outer join
        member members1_ 
            on hospital0_.hospital_id=members1_.hospitals_id 
    left outer join
        equipment equipments2_ 
            on members1_.member_id=equipments2_.members_id 
    order by
        hospital0_.hospital_name,
        members1_.member_id asc

N+1문제가 사라졌다.

그런데 사실 Set으로 하기보다는 그냥 다중 엔티티 그래프를 사용하지 않는게 나은것 같다.
왜냐하면 Set은 중복을 제거해주는데 쿼리 할때마다 중복 제거 연산이 발생해서 오버헤드가 나기 때문이다..
이 문제를 어떻게 해결하는게 맞는지 잘 모르겠다.

References

https://stackoverflow.com/questions/4334970/hibernate-throws-multiplebagfetchexception-cannot-simultaneously-fetch-multipl
https://programmer93.tistory.com/83
https://velog.io/@mingsound21/EntityGraph