更新时间:2023-12-02 09:02:58
如果在链接表 (STUDENT_COURSE) 上添加额外的字段,则必须根据 skaffman 的回答或其他方法选择一种方法,如下所示.
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.
有一种方法,其中链接表 (STUDENT_COURSE) 的行为类似于 @Embeddable,根据:
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
}
}
因此您将同时拥有学生和课程课程
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>();
}
记住:@Embeddable 类的生命周期绑定到拥有的实体类(Student 和 Course),所以要注意它.
remember: @Embeddable class has its lifecycle bound to the owning entity class (Both Student and Course), so take care of it.
建议:由于@ManyToMany 映射的一些限制 - 级联操作,Hibernate 团队支持这两种方法(@OneToMany(skaffman 的回答)或@CollectionsOfElements).
advice: Hibernate team suppports these two approachs (@OneToMany (skaffman's answer) or @CollectionsOfElements) due some limitations in @ManyToMany mapping - cascade operation.
问候,