SpringDataJPA：出了什么問題或如何更正此jpql查詢-有解無憂

物體關系是 - 一個學生屬于一所大學，一個大學可以有多個學生。所以 Student --> College 之間存在 ManyToOne 關系，College --> Student 之間存在 OneToMany 關系。

物體如下。

@Entity
public class College {

    @Id
    @GeneratedValue
    private int collegeId;

    private String collegeName;

    @OneToMany(targetEntity = Student.class, mappedBy = "college") 
    private List<Student> students;

和

@Entity
public class Student {

    @Id
    @GeneratedValue
    private int studentId;

    private String studentName;

    @ManyToOne
    @JoinColumn(name = "collegeId_fk")
    private College college;

1）我在spring資料jpa存盤庫中使用下面的jpql查詢。

@Query("SELECT c FROM College c LEFT JOIN FETCH c.students where c.collegeId IN (2)")
public List<College> findByCollegeIdsJPQL();

我期望一個包含單個大學物體的串列作為 CollegeId 回傳是大學物體的主鍵，我只為IN. 但是我得到的是所有具有相同主鍵（collegeId = 2）的大學串列。回傳串列的大小等于學院的學生人數。

System.err.println("collegeRepo.findByCollegeIdsJPQL().size(): "   collegeRepo.findByCollegeIdsJPQL().size()); 
//output: collegeRepo.findByCollegeIdsJPQL().size(): 6

而對于

collegeRepo.findByCollegeIdsJPQL().forEach( System.err::println );

輸出：
學院 [collegeId=2，collegeName=college2]
學院 [collegeId=2，collegeName=college2]
學院 [collegeId=2，collegeName=college2]
學院 [collegeId=2，collegeName=college2]
學院 [collegeId=2，collegeName= College2]
學院 [collegeId=2，collegeName=college2]

2) 我注意到的另一個問題，
第二次呼叫collegeRepo.findByCollegeIdsJPQL()導致另一個 sql 查詢，即 DB 命中。不能從會話（第一級）快取中提供此服務。我已經用

  @Transactional
  public void run(String... args) throws Exception {

輸出日志 -

2022-02-20 15:10:59.140[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_0__] : [INTEGER]) - [6]
collegeRepo.findByCollegeIdsJPQL().size(): 6
[2m2022-02-20 15:10:59.153[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.s.t.i.TransactionInterceptor          [0;39m [2m:[0;39m No need to create transaction for [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findByCollegeIdsJPQL]: This method is not transactional.
Hibernate: 
    select
        college0_.college_id as college_1_1_0_,
        students1_.student_id as student_1_12_1_,
        college0_.college_name as college_2_1_0_,
        students1_.college_id_fk as college_3_12_1_,
        students1_.student_name as student_2_12_1_,
        students1_.college_id_fk as college_3_12_0__,
        students1_.student_id as student_1_12_0__ 
    from
        college college0_ 
    left outer join
        student students1_ 
            on college0_.college_id=students1_.college_id_fk 
    where
        college0_.college_id in (
            2
        )
[2m2022-02-20 15:10:59.245[0;39m [32m INFO[0;39m [35m18416[0;39m [2m---[0;39m [2m[on(3)-127.0.0.1][0;39m [36mo.a.c.c.C.[Tomcat].[localhost].[/]      [0;39m [2m:[0;39m Initializing Spring DispatcherServlet 'dispatcherServlet'
[2m2022-02-20 15:10:59.246[0;39m [32m INFO[0;39m [35m18416[0;39m [2m---[0;39m [2m[on(3)-127.0.0.1][0;39m [36mo.s.web.servlet.DispatcherServlet       [0;39m [2m:[0;39m Initializing Servlet 'dispatcherServlet'
[2m2022-02-20 15:10:59.247[0;39m [32m INFO[0;39m [35m18416[0;39m [2m---[0;39m [2m[on(3)-127.0.0.1][0;39m [36mo.s.web.servlet.DispatcherServlet       [0;39m [2m:[0;39m Completed initialization in 1 ms
[2m2022-02-20 15:10:59.403[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [1]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.404[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.405[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [3]
[2m2022-02-20 15:10:59.405[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.405[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [4]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [5]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_1_1_0_] : [INTEGER]) - [2]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([student_1_12_1_] : [INTEGER]) - [6]
[2m2022-02-20 15:10:59.406[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.h.type.descriptor.sql.BasicExtractor  [0;39m [2m:[0;39m extracted value ([college_3_12_0__] : [INTEGER]) - [2]
College [collegeId=2, collegeName=college2]
College [collegeId=2, collegeName=college2]
College [collegeId=2, collegeName=college2]
College [collegeId=2, collegeName=college2]
College [collegeId=2, collegeName=college2]
College [collegeId=2, collegeName=college2]
[2m2022-02-20 15:10:59.407[0;39m [32mTRACE[0;39m [35m18416[0;39m [2m---[0;39m [2m[           main][0;39m [36mo.s.t.i.TransactionInterceptor          [0;39m [2m:[0;39m Completing transaction for [com.demo.MyRunner.run]

uj5u.com熱心網友回復：

(1) 因為LEFT JOIN將回傳大學及其學生的組合。因此，如果一所大學有 N 個學生，則將回傳 N 條記錄。您必須添加DISTINCT以洗掉重復項：

@Query("SELECT distinct c FROM College c LEFT JOIN FETCH c.students where c.collegeId IN (2)")
public List<College> findByCollegeIdsJPQL();

但是，它會導致生成的 SQL hasdistinct關鍵字，這可能會對性能產生影響。因此，從 Hibernate 5.2 開始，他們提供了一個名為的查詢提示hibernate.query.passDistinctThrough，可以配置不在distinct生成的 SQL 中添加關鍵字，而 Hibernate 將有助于洗掉重復項：

@Query("SELECT distinct c FROM College c LEFT JOIN FETCH c.students where c.collegeId IN (2)")
@QueryHints(@QueryHint(name = org.hibernate.jpa.QueryHints.HINT_PASS_DISTINCT_THROUGH, value = "false"))
public List<College> findByCollegeIdsJPQL();

有關詳細資訊，請參閱此博客文章。

(2) 正常。您必須配置二級快取和查詢快取以防止另一個資料庫命中。一級快取主要EntityManager#get()用于在同一事務中通過 ID 獲取物體時使用。它在使用 JPQL 查詢時不起作用。

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標籤：爪哇春天休眠 jpa 弹簧数据-jpa

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