Java 可以向@ManyToMany Hibernate 额外表添加额外字段吗？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/1153409/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Can add extra field(s) to @ManyToMany Hibernate extra table?
提问by Am1rr3zA
I have these two class(table)
我有这两个类（表）
@Entity
@Table(name = "course")
public class Course {
@Id
@Column(name = "courseid")
private String courseId;
@Column(name = "coursename")
private String courseName;
@Column(name = "vahed")
private int vahed;
@Column(name = "coursedep")
private int dep;
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "student_course", joinColumns = @JoinColumn(name = "course_id"), inverseJoinColumns = @JoinColumn(name = "student_id"))
private Set<Student> student = new HashSet<Student>();
//Some setter and getter
and this one:
和这个：
@Entity
@Table(name = "student")
public class Student {
@Id
@Column(name="studid")
private String stId;
@Column(nullable = false, name="studname")
private String studName;
@Column(name="stmajor")
private String stMajor;
@Column(name="stlevel", length=3)
private String stLevel;
@Column(name="stdep")
private int stdep;
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "student_course"
,joinColumns = @JoinColumn(name = "student_id")
,inverseJoinColumns = @JoinColumn(name = "course_id")
)
private Set<Course> course = new HashSet<Course>();
//Some setter and getter
After running this code an extra table was created in database(student_course), now I wanna know how can I add extra field in this table like (Grade, Date , and ... (I mean student_course table)) I see some solution but I don't like them, Also I have some problem with them:
运行此代码后，在数据库（student_course）中创建了一个额外的表，现在我想知道如何在该表中添加额外的字段，例如（Grade、Date 和 ...（我的意思是 student_course 表））我看到了一些解决方案，但是我不喜欢它们，而且我对它们有一些问题：
采纳答案by Arthur Ronald
If you add extra fields on a linked table (STUDENT_COURSE), you have to choose an approach according to skaffman's answer or another as shown bellow.
如果您在链接表 (STUDENT_COURSE) 上添加额外字段，则必须根据 skaffman 的答案或其他方法选择一种方法，如下所示。
There is an approach where the linked table (STUDENT_COURSE) behaves like a @Embeddable according to:
有一种方法，其中链接表 (STUDENT_COURSE) 的行为类似于 @Embeddable，根据：
@Embeddable
public class JoinedStudentCourse {
// Lets suppose you have added this field
@Column(updatable=false)
private Date joinedDate;
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="STUDENT_ID", insertable=false, updatable=false)
private Student student;
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="COURSE_ID", insertable=false, updatable=false)
private Course course;
// getter's and setter's
public boolean equals(Object instance) {
if(instance == null)
return false;
if(!(instance instanceof JoinedStudentCourse))
return false;
JoinedStudentCourse other = (JoinedStudentCourse) instance;
if(!(student.getId().equals(other.getStudent().getId()))
return false;
if(!(course.getId().equals(other.getCourse().getId()))
return false;
// ATT: use immutable fields like joinedDate in equals() implementation
if(!(joinedDate.equals(other.getJoinedDate()))
return false;
return true;
}
public int hashcode() {
// hashcode implementation
}
}
So you will have in both Student and Course classes
因此，您将在 Student 和 Course 类中都有
public class Student {
@CollectionOfElements
@JoinTable(
table=@Table(name="STUDENT_COURSE"),
joinColumns=@JoinColumn(name="STUDENT_ID")
)
private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();
}
public class Course {
@CollectionOfElements
@JoinTable(
table=@Table(name="STUDENT_COURSE"),
joinColumns=@JoinColumn(name="COURSE_ID")
)
private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();
}
remember: @Embeddable class has its lifecycle bound to the owning entity class (Both Student and Course), so take care of it.
请记住：@Embeddable 类的生命周期绑定到拥有的实体类（Student 和 Course），因此请注意它。
advice: Hibernate team suppports these two approachs (@OneToMany (skaffman's answer) or @CollectionsOfElements) due some limitations in @ManyToMany mapping - cascade operation.
建议：Hibernate 团队支持这两种方法（@OneToMany（skaffman 的回答）或 @CollectionsOfElements），因为在 @ManyToMany 映射中存在一些限制 - 级联操作。
regards,
问候，
回答by skaffman
The student_course table is there purely to record the association between the two entities. It is managed by hibernate, and can contain no other data.
student_course 表纯粹是为了记录两个实体之间的关联。它由休眠管理，不能包含其他数据。
The sort of data you want to record needs to be modelled as another entity. Perhaps you could a one-to-many association between Course and StudentResult (which contains the grade, etc), and then a many-to-one association between StdentResult and Student.
您要记录的数据类型需要建模为另一个实体。也许您可以在 Course 和 StudentResult（包含成绩等）之间建立一对多关联，然后在 StdentResult 和 Student 之间建立多对一关联。
回答by mcintyre321
Drop the many-to-many, create a class called StudentCourseRelationship and set up one to manys on Student and Course to the StudentCourseRelationship.
删除多对多，创建一个名为 StudentCourseRelationship 的类，并在 Student 和 Course 上设置一对多到 StudentCourseRelationship。
You can put all sorts of things on it, like DateEnrolled, DateKickedOut etc. etc.
你可以在上面放各种各样的东西，比如 DateEnrolled、DateKickedOut 等等。
IMO the many-to-many mapping is a bit of a con.
IMO 多对多映射有点骗人。
回答by azerole
The accepted answer unfortunately doesn't work for me, hibernate generates the join table in a weird way (all join columns are duplicated). However the variant with dedicated entity for the join table works fine. Here it is described in great detail: http://www.mkyong.com/hibernate/hibernate-many-to-many-example-join-table-extra-column-annotation/
不幸的是，接受的答案对我不起作用，hibernate 以一种奇怪的方式生成连接表（所有连接列都是重复的）。然而，连接表专用实体的变体工作正常。这里有很详细的描述：http: //www.mkyong.com/hibernate/hibernate-many-to-many-example-join-table-extra-column-annotation/
