無法獲取連接延遲初始化 collections

Question

由於 MultipleBagFetchException，我無法執行簡單的 fetchjoin。

@Entity
public class Person {

    @OneToMany(mappedBy="person",fetch=FetchType.LAZY)
    private List<Auto> autos;    
}


@Entity
public class Auto {

    @ManyToOne
    @JoinColumn(name = "person_id", nullable = false)
    private Person person;

   @OneToMany(mappedBy="auto",fetch=FetchType.LAZY)
   private List<Tool> tools;

}

@Entity
@Table(name="tool")
public class Tool {

    @ManyToOne
    @JoinColumn(name = "auto_id", nullable = false)
    private Auto auto;
}

如您所見，我所有的關聯都使用默認的 fetchtype。

 @Query("SELECT p FROM Person p JOIN FETCH p.autos a JOIN FETCH a.tools")
 List<Person>findAll();

結果：

Caused by: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags: [com.example.entities.Person.autos, com.example.entities.Auto.tools]

我已閱讀有關此異常的信息，但在這些情況下，此異常的原因是 collections 使用了 EAGER 提取類型。 那這個呢？ 這是最簡單的實體關系。

最重要的是，假設我們不允許觸摸實體。 如何僅在查詢端解決這個問題？

Answer 1

有一種方法可以在不觸及實體的情況下避免 n+1 次查詢，只需更改 findAll 的查詢即可。 我們可以編寫一個包裝器 function，它將首先加載帶有汽車的人員，然后他們在單個 select 中獲取所有工具。

個人資料庫

    @Query("SELECT distinct p FROM Person p JOIN FETCH p.autos a")
    List<Person> findAll();

包裝代碼

    List<Person> persons = personRepository.findAll();
    Session session = (Session) entityManager.getDelegate();
    List<Auto> autos = new ArrayList<>();
    for (Person person : persons) {
        if(!CollectionUtils.isEmpty(person.getAutos())) {
            autos.addAll(person.getAutos());
        }
    }
    try{
        autos = session.createQuery("select distinct a from Auto a Join fetch a.tools " +
                " where a in :autos", Auto.class)
                .setParameter("autos", autos)
                .setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
                .getResultList();
    } catch (Exception ex) {
        ex.printStackTrace();
    }

第一個查詢將是：

SELECT DISTINCT
          person0_.id       AS id1_6_0_,
          autos1_.id        AS id1_0_1_,
          person0_.name     AS name2_6_0_,
          autos1_.name      AS name2_0_1_,
          autos1_.person_id AS person_i3_0_1_,
          autos1_.person_id AS person_i3_0_0__,
          autos1_.id        AS id1_0_0__
FROM
          Person person0_
          INNER JOIN
                    Auto autos1_
          ON
                    person0_.id=autos1_.person_id

生成的第二個查詢將是：

    SELECT
          auto0_.id        AS id1_0_0_,
          tools1_.id       AS id1_8_1_,
          auto0_.name      AS name2_0_0_,
          auto0_.person_id AS person_i3_0_0_,
          tools1_.auto_id  AS auto_id3_8_1_,
          tools1_.name     AS name2_8_1_,
          tools1_.auto_id  AS auto_id3_8_0__,
          tools1_.id       AS id1_8_0__
FROM
          Auto auto0_
          INNER JOIN
                    Tool tools1_
          ON
                    auto0_.id=tools1_.auto_id
WHERE
          auto0_.id IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)

除此之外，我相信我們的選項是有限的，我們將不得不更改工具實體 FetchMode 或為默認 FetchMode.SELECT 添加 BatchSize 以便在單獨的查詢中獲取工具。

    @OneToMany(mappedBy = "auto", fetch = FetchType.LAZY)
    @Fetch(FetchMode.SUBSELECT)
    private List<Tool> tools;

查詢將是

SELECT
          tools0_.auto_id AS auto_id3_8_1_
        , tools0_.id      AS id1_8_1_
        , tools0_.id      AS id1_8_0_
        , tools0_.auto_id AS auto_id3_8_0_
        , tools0_.name    AS name2_8_0_
FROM
          Tool tools0_
WHERE
          tools0_.auto_id IN
          (
                    SELECT
                              autos1_.id
                    FROM
                              Person person0_
                              INNER JOIN
                                        Auto autos1_
                              ON
                                        person0_.id=autos1_.person_id
          )