Question

我只是想了解如何在Hibernate中映射以下情况。

有一些课程和一些学生。任何学生都有自己的学习计划，其中包含选修课程和必修课程。

我很容易在数据库世界中对此进行建模。我会有四张桌子：

课程（身份证，姓名，说明）
学生（身份证，姓名）
student_course_optional（student_id，course_id）
student_course_required（student_id，course_id）

但我真的不知道如何用hibernate映射它。

这是我的初稿：

@Entity
public class Student {

    private Long id;
    private Long version;
    private String name;
    private List<Course> requiredCourses;
    private List<Course> optionalCourses;

    ...

    @ManyToMany
    @JoinTable(name="Course")
    public List<Course> getRequiredCourses() {
        return requiredCourses;
    }

    @ManyToMany
    @JoinTable(name="Course")
    public List<Course> getOptionalCourses() {
        return optionalCourses;
    }
}

@Entity
public class Course {

    private Long id;
    private Long version;
    private String name;
    private List<Student> students;
    private List<Student> optionalStudents;

    ...

    @ManyToMany(mappedBy="requiredCourses")
    public List<Course> getStudents() {
        return requiredCourses;
    }

    @ManyToMany(mappedBy="optionalCourses")
    public List<Course> getOptionalStudents() {
        return optionalCourses;
    }
}

但不知怎的，这对我来说很奇怪。或者这是正确的吗？

Answer 1

示例实现可以使用中间实体，例如StudyPlan，其充当UML association class。这样的实体包括：

由StudyPlanId class
由student和course字段代表的外键（在JPA术语中，这是派生标识符）
由optional字段表示的其他状态（它允许区分必需和可选课程）

@Entity
@IdClass(StudyPlanId.class)
public class StudyPlan {
    @Id
    @ManyToOne
    private Student student;

    @Id
    @ManyToOne
    private Course course;

    private boolean optional;
    ...
}

public class StudyPlanId implements Serializable {
    private int course;
    private int student;

    @Override
    public int hashCode() { ... }
    @Override
    public boolean equals(Object obj) { ... }
    ... 
}

@Entity
public class Student {
    @Id
    private int id;

    @OneToMany(mappedBy = "student")
    private Collection<StudyPlan> plan;

    private String name;
    ...
}

@Entity
public class Course {
    @Id
    private int id;

    @OneToMany(mappedBy = "course")
    private Collection<StudyPlan> plan;

    private String name;
    private String description;
    ...     
}

上述实体模型将产生以下数据模型：

Student            StudyPlan            Course
===============    =================    ===============
id           PK    student_id  FK PK    id           PK
name               course_id   FK PK    name      
                   optional             description

Answer 2

@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "student_course_required", joinColumns = @JoinColumn(name = "student_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "course_id", referencedColumnName = "id"))
private List<Course> requiredCourses= new ArrayList<Course>();

@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "student_course_optional", joinColumns = @JoinColumn(name = "student_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "course_id", referencedColumnName = "id"))
private List<Course> optionalCourses= new ArrayList<Course>();

Public Class Student需要更改代码。

 @ManyToMany(fetch = FetchType.EAGER)
 @JoinTable(name = "student_course_required", joinColumns = @JoinColumn(name = "course_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "student_id", referencedColumnName = "id"))
 private List<Student> requiredStudents= new ArrayList<Student>();

 @ManyToMany(fetch = FetchType.EAGER)
 @JoinTable(name = "student_course_optional", joinColumns = @JoinColumn(name = "course_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "student_id", referencedColumnName = "id"))
 private List<Student> optionalStudents= new ArrayList<Student>();

public Class Course需要更改代码。我认为这可以解决你的问题。

休眠。如何将两个多对多映射到同一个实体

2 个答案: