Spring 數據 JPA：有什么問題或如何更正此 jpql 查詢

Question

實體關系是 - 一個學生屬於一所大學，一所大學可以有多個學生。 所以在 Student --> College 之間存在 ManyToOne 關系，在 College --> Student 之間存在 OneToMany 關系。

實體如下。

@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 數據 jpa 存儲庫中使用以下 jpql 查詢。

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

我希望返回一個包含單個大學實體的列表，因為 collegeId 是大學實體的主鍵，我只為IN提供一個 id。 但我得到的是所有具有相同主鍵 (collegeId=2) 的大學列表。 返回列表的大小等於學院的學生人數。

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

而對於

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

output：
大學 [collegeId=2, collegeName=college2]
大學 [collegeId=2, collegeName=college2]
大學 [collegeId=2, collegeName=college2]
大學 [collegeId=2, collegeName=college2]
大學 [collegeId=2, collegeName=college2]
大學 [collegeId=2, collegeName=college2]

2）我注意到的另一個問題，
第二次調用collegeRepo.findByCollegeIdsJPQL()會導致另一個 sql 查詢，即數據庫命中。 這不能從會話（第一級）緩存中提供。 我已經注釋了調用方法

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

output 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]

Answer 1

(1) 因為LEFT JOIN將返回大學及其學生的組合。 因此，如果一所大學有 N 個學生，則將返回 N 條記錄。 您必須添加DISTINCT以刪除重復項：

@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 具有distinct關鍵字，這可能會對性能產生影響。 因此，自 Hibernate 5.2 以來，他們提供了一個名為hibernate.query.passDistinctThrough的查詢提示，它可以配置不在生成的 SQL 中添加distinct關鍵字，而 Hibernate 將有助於刪除重復項：

@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();

有關詳細信息，請參閱此博客文章。

(2) 正常。 您必須配置二級緩存和查詢緩存以防止再次命中數據庫。 一級緩存主要在同一事務中使用EntityManager#get()通過 ID 獲取實體時起作用。 它在使用 JPQL 查詢時不起作用。