LEFT JOIN FETCH 不起作用 - Spring 数据 JPA

Question

我在使用 LEFT JOIN FETCH 时遇到问题。请参考我下面的实体和存储库。

考虑一个场景，选修课程在任何时候都不会从表格中删除。但是可以删除学生信息。 数据库中的两个表之间没有主键和外键关系。只是我们有共同的列“STUDENT_ID”。

@NamedEntityGraphs({
@NamedEntityGraph(
        name = "Student.optionalCourse",
        attributeNodes = @NamedAttributeNode("optionalCourse")
    )
})
@Table("STUDENT")
class Student {

    @Id
    @column_name(STUDENT_ID)
    private integer studentId;

    @column_name(STUDENT_AGE)
    private int studentAge;

    @OneToMany(fetch = FetchType.LAZY, mappedBy="student")
      private List<OptionalCourse> optionalCourse;

}


@NamedEntityGraphs({
@NamedEntityGraph(
        name = "OptionalCourse.student",
        attributeNodes = @NamedAttributeNode("student")
    )
})
@Table("OPTIONAL_COURSES")
class OptionalCourse {

    @Id
    @column_name(ID)
    private Integer id;

    @column_name(STUDENT_ID)
    private Integer studentId;

    @column_name(SUBJECT_NAME)
    private String subjectName;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "STUDENT_ID" , referencedColumnName="STUDENT_ID", insertable = false, updatable = false )
    private Student student;

}


@Repository
@Transactional(rollbackFor = Exception.class)
public interface OptionalCourseRepository extends JpaRepository<OptionalCourse,Integer> {

  @Query(value = "SELECT oc FROM OptionalCourse oc LEFT JOIN FETCH oc.student where oc.studentId > :studentId",
      countQuery = "SELECT count(oc.id) FROM OptionalCourse oc where oc.studentId > :studentId")
  public Page<OptionalCourse> findOptionalSubjectPagesByStudentId(@Param("studentId") Integer studentId, Pageable pageable);

}

没有事务的服务方法：

public List<CustomBeanClass> retrieveRpdLogRequestsList(Integer studentId, Integer pageIndex, Integer pageResultsSize) {

    Pageable pageWithIndexAndSize = Pageable.unpaged();
    
    if(Objects.nonNull(pageIndex) && Objects.nonNull(pageResultsSize) && pageIndex >= 0 && pageResultsSize > 0) {
      
      pageWithIndexAndSize = PageRequest.of(pageIndex, pageResultsSize);
    }
    
    Page<OptionalCourse> pageData = optionalCourseRepository.findOptionalSubjectPagesByStudentId(studentId, pageWithIndexAndSize);
    
    List<OptionalCourse> pageDataList = pageData.toList();

    pageDataList.forEach(course -> {

          OptionalCourse oc = course;

          //When trying to fetch student info from optional course, when student info is not there then we are getting below error
          oc.getStudent(); **//com.sun.jdi.InvocationException: Exception occurred in target VM occurred invoking method hibernate & org.hibernate.LazyInitializationException: could not initialize proxy [com.xxx.xxx.Student#550] - no Session**
    });
}

事务性的服务方法：

@Transactional(readOnly = true, propagation = Propagation.REQUIRED)
public List<CustomBeanClass> retrieveRpdLogRequestsList(Integer studentId, Integer pageIndex, Integer pageResultsSize) {

    Pageable pageWithIndexAndSize = Pageable.unpaged();
    
    if(Objects.nonNull(pageIndex) && Objects.nonNull(pageResultsSize) && pageIndex >= 0 && pageResultsSize > 0) {
      
      pageWithIndexAndSize = PageRequest.of(pageIndex, pageResultsSize);
    }
    
    Page<OptionalCourse> pageData = optionalCourseRepository.findOptionalSubjectPagesByStudentId(studentId, pageWithIndexAndSize);
    
    List<OptionalCourse> pageDataList = pageData.toList();

    pageDataList.forEach(course -> {

          OptionalCourse oc = course;

          //When trying to fetch student info from optional course, when student info is not there then we are getting below error
          oc.getStudent(); **// EntityNotFoundException - Its trying to fetch the dependent entity student here**
    });
}

即使我使用了 LEFT JOIN FETCH，为什么当我调用依赖实体（即）oc.getStudent();生成的查询具有正确的语法为 LEFT OUTER JOIN，在 SQLDeveloper 中它给出了预期的结果。我们正在使用 Oracle 数据库。

我犯了什么错误？我不被允许使用 EAGER FETCH 策略。请通过 Lazy Fetch 本身帮助解决此问题。提前谢谢！！！！

Answer 1

我猜你正在使用在 Spring 中默认启用的 OSIV（视图中打开会话）反模式？这是该方法的常见问题。问题是您的第一级缓存（持久性上下文）包含具有学生字段未初始化代理的实体。现在，即使您再次选择该实体并加入 fetch 关联，Hibernate 也不会替换该对象，因为它必须保留对象身份并且也不会替换代理。要解决这个问题，您必须在使用 EntityManager.clear() 进行查询之前清除持久性上下文