可以添加额外的字段到@ManyToMany Hibernate额外的表？ [英] Can add extra field(s) to @ManyToMany Hibernate extra table?
问题描述
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））
私人设置< Student> student = new HashSet< Student>（）;
//一些setter和getter
和这一个：
@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>（）;
//一些setter和getter
运行此代码后，数据库中创建了一个额外的表（student_course），现在我想知道如何在表格中添加额外的字段，例如（年级，日期和...（我的意思是student_course表））
我看到一些解决方案，但我不喜欢它们，我也有一些问题：
根据以下方法，链接表（STUDENT_COURSE）的行为类似于@Embeddable：
@Embeddable
public class JoinedStudentCourse {
//假设你已经添加了这个字段
@列（updatable = false）
私人日期joinedDate;
@ManyToOne（fetch = FetchType.LAZY）
@JoinColumn（name =STUDENT_ID，insertable = false，updatable = false）
私立学生;
$ b @ManyToOne（fetch = FetchType.LAZY）
@JoinColumn（name =COURSE_ID，insertable = false，updatable = false）
私人课程;
// getter's和setter's
public boolean equals（Object instance）{
if（instance == null）
return false;
if（！（instance instanceof JoinedStudentCourse））
return false;
JoinedStudentCourse other =（JoinedStudentCourse）instance; $！$ b $ if（！（student.getId（）。equals（other.getStudent（）。getId（）））
return false;
if（！（course.getId ）.equals（other.getCourse（）。getId（）））
返回false;
// ATT：在equals（）中使用诸如joinedDate这样的不可变字段实现
if ！（joinedDate.equals（other.getJoinedDate（）））
返回false;
返回true;
}
public int hashcode（）{
//哈希码实现
}
}
所以你必须在学生和课程课上
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设置< JoinedStudentCourse> joined = new HashSet< JoinedStudentCourse>（）;
$ / code> 请记住：@Embeddable类的生命周期必须拥有
建议：Hibernate团队支持这两种方法（@OneToMany（skaffman's answer）或@CollectionsOfElements）一些限制@ManyToMany映射 - 级联操作。
问候，
I have these two class(table)
@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:
@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:
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.
There is an approach where the linked table (STUDENT_COURSE) behaves like a @Embeddable according to:
@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
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.
advice: Hibernate team suppports these two approachs (@OneToMany (skaffman's answer) or @CollectionsOfElements) due some limitations in @ManyToMany mapping - cascade operation.
regards,
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