如何解决休眠 n+1 问题 OneToMany 和 ManyToOne答案

【问题标题】：How to resolve hibernate n+1 problem OneToMany and ManyToOne如何解决休眠 n+1 问题 OneToMany 和 ManyToOne
【发布时间】：2021-04-26 02:14:33
【问题描述】：

我遇到了 N+1 Hibernate 的问题。我有以下实体：

public class Coupon {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "ID")
    private Long id;

    @Builder.Default
    @OneToMany(fetch = FetchType.LAZY,
            targetEntity = CouponType.class,
            cascade = CascadeType.ALL,
            mappedBy = "coupon")
    private List<CouponType> couponTypeList = new ArrayList<>();

public class CouponType {

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


    @ManyToOne
    private Match match;

    @ManyToOne
    private Coupon coupon;

public class Match {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "ID")
    private Long id;

    @Builder.Default
    @OneToMany(fetch = FetchType.LAZY,
            targetEntity = CouponType.class,
            mappedBy = "match")
    private List<CouponType> couponTypeList = new ArrayList<>();

而且我想避免休眠中的 N+1 问题。如何在 JPQL 中正确编写查询？

解决方案：

@Query("select c from Coupon c join fetch c.couponTypeList t join fetch t.match where c.id = ?1") 
Optional<Coupon> getCoupon(Long couponId)

【问题讨论】：

标签： java hibernate jpql

【解决方案1】：

尝试使用以下查询：

@Query("select c from Coupon c join fetch c.couponTypeList where c.id = ?1")
Optional<Coupon> getCoupon(Long couponId);

【讨论】：

@Query("select c from Coupon c join fetch c.couponTypeList t join fetch t.match where c.id = ?1") 可选 getCoupon(Long couponId);
当您尝试使用 `join fetch` 进行多个集合提取时，请不要遇到MultipleBagFetchException。看到这个question。