如何将 2 个字段映射到同一个多对多关联？答案

【问题标题】：How map 2 fields to the same Many to Many association?如何将 2 个字段映射到同一个多对多关联？
【发布时间】：2021-10-30 11:46:00
【问题描述】：

有一个 Spring JPA 应用程序：

对于一个用例，我想将所有学生数据读入一组学生，如下面的第一组 (CourseRepository.findAll()) - 这仅用于读取与课程相关联的学生。

对于第二个用例，我想仅使用学生 UUID 来读取和更新学生集（因为这明显更快，避免了许多连接/级联） - 如下面的第二个集 - 这仅用于添加或将学生删除到课程中。

@Entity
@Table(...)
public class Course {
    @Id
    private UUID id;

   @ManyToMany(fetch = FetchType.LAZY, cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH, CascadeType.PERSIST})
   @JoinTable(
       schema = "...",
       name = "...",
       joinColumns = {@JoinColumn(name = "course_id", referencedColumnName = "id")},
       inverseJoinColumns = {@JoinColumn(name = "student_id", referencedColumnName = "id")}
   )
   private Set<Student> students = new java.util.HashSet<>();

   @ManyToMany
   @JoinTable(
       schema = "...",
       name = "...",
       joinColumns = {@JoinColumn(name = "course_id", referencedColumnName = "id")},
       inverseJoinColumns = {@JoinColumn(name = "student_id", referencedColumnName = "id")}
    )
   private Set<UUID> students = new java.util.HashSet<>();
   ...
}

@Entity
@Table(name="student")
public class Student {
    @Id
    private UUID id;

    private String name;
...
}

@Repository
public interface CourseRepository extends JpaRepository<Course, UUID> {}

【问题讨论】：

标签： hibernate jpa spring-data-jpa hibernate-mapping

【解决方案1】：

您应该将多对多关联建模为两个一对多关联，因为无论如何这都是您最终需要的。这样，可以通过简单地持久化这个新的CourseStudentAssignment 实体来完成分配。

@Entity
public class CourseStudentAssignment {
    @EmbeddedId
    CourseStudentAssignmentId id;
    @ManyToOne(fetch = LAZY)
    @MapsId("courseId")
    Course course;
    @ManyToOne(fetch = LAZY)
    @MapsId("studentId")
    Student student;

}
@Embeddable
public class CourseStudentAssignmentId {
    UUID courseId;
    UUID studentId;
}
@Entity
@Table(...)
public class Course {
    @Id
    private UUID id;

   @OneToMany(mappedBy = "course", fetch = FetchType.LAZY, cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH, CascadeType.PERSIST})
   private Set<CourseStudentAssignment> studentAssignments = new java.util.HashSet<>();

}
@Entity
@Table(...)
public class Student {
    @Id
    private UUID id;

   @OneToMany(mappedBy = "student", fetch = FetchType.LAZY, cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH, CascadeType.PERSIST})
   private Set<CourseStudentAssignment> courseAssignments = new java.util.HashSet<>();

}

【讨论】：