在这里，我正在尝试 JPA 中的多对多关系，我创建了表“tblcourse”和“tblStudent”，学生可以注册许多课程，
create table tblcourse(
id integer primary key,
name varchar(100),
duration integer
);
create table tblcourseStudent(
studentid integer references tblstudent(studentId),
courseId integer references tblcourse(id),
constraint pk_composit_cs primary key(studentid,courseId)
)
Create table tblStudent(
studentId integer primary key,
……..
….
);
上述关系的JPA表示如下，这是StudentEntity.java的代码，
@Entity
@Table(name="TBLSTUDENT")
public class StudentEntity implements Serializable{
private static final long serialVersionUID = 100034222342L;
@Id
@Column(name="STUDENTID")
private Integer studentId;
@Column(name="STUDENTNAME")
private String studentName;
@Column(name="CONTACTNO")
private String contactNumber;
@Embedded
private StudentAddress address;
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="DEPTID")
private DeptEntity deptEntity;
@ManyToMany(fetch=FetchType.LAZY)
@JoinTable(name="tblcourseStudent",
joinColumns=@JoinColumn(name="studentid"),
inverseJoinColumns=@JoinColumn(name="courseId"))
private List<CourseEntity> courseList;
....
.....
.....
}
这是 CourseEntity.java 的代码，
@Entity
@Table(name="TBLCOURSE")
public class CourseEntity implements Serializable{
public CourseEntity(){
}
public CourseEntity(Integer courseId,String courseName,Integer courseDuration){
this.courseId = courseId;
this.courseName = courseName;
this.courseDuration = courseDuration;
}
/**
*
*/
private static final long serialVersionUID = -2192479237310864341L;
@Id
@Column(name="ID")
private Integer courseId;
@Column(name="NAME")
private String courseName;
@Column(name="DURATION")
private Integer courseDuration;
@ManyToMany(fetch=FetchType.LAZY)
@JoinTable(name="tblcourseStudent",
joinColumns=@JoinColumn(name="courseId"),
inverseJoinColumns=@JoinColumn(name="studentid"))
private List<StudentEntity> studentList;
.........
}
现在，当我尝试通过 StudentEntity.java 插入课程时，后端触发的 SQL 查询是
delete
from
tblcourseStudent
where
studentid=?
insert
into
tblcourseStudent
(studentid, courseId)
values
(?, ?)
insert
into
tblcourseStudent
(studentid, courseId)
values
(?, ?)
而且，当我尝试通过 CourseEntity.java 插入学生时，触发的 SQL 查询如下，
delete
from
tblcourseStudent
where
courseId=?
insert
into
tblcourseStudent
(courseId, studentid)
values
(?, ?)
在我的两种情况下，记录都被删除，然后新的映射被插入。因此，如果我为学生插入课程，首先将从第三个表中删除该学生的所有先前课程，然后输入新课程，
所以，我的问题是，如果我不想删除旧课程并为学生添加新课程，我该如何实现，即我想保留旧的关系，
天气我必须以编程方式实现这一点，或者我已经更改了注释，等待回复
当我们将单个学生映射到多个课程时，会调用 StudentServiceBean.java 中编写的代码和方法“mapStudentToCourses”
@Stateless
@TransactionManagement(TransactionManagementType.CONTAINER)
public class StudentServiceBean implements StudentService{
@PersistenceContext(unitName="forPractise")
private EntityManager entityMgr;
@Resource
private SessionContext sessionContext;
@EJB
private DeptService deptService;
..........
......
...
@Override
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void mapStudentToCourses(Integer studentId,String courseIdList) throws Exception{
List<CourseEntity> courseList = null;
StudentEntity studentEntity = null;
TypedQuery<CourseEntity> courseQuery = null;
String query = "select c from CourseEntity c where c.courseId in ("+courseIdList+")";
try{
courseQuery = entityMgr.createQuery(query,CourseEntity.class);
courseList = courseQuery.getResultList();
studentEntity = entityMgr.find(StudentEntity.class, studentId);
studentEntity.setCourseList(courseList);
entityMgr.merge(studentEntity);
}catch(Exception e){
sessionContext.setRollbackOnly();
throw e;
}
}
这是一个课程映射到多个学生时的代码，它的 CourseServiceBean.java
@Stateless
@TransactionManagement(TransactionManagementType.CONTAINER)
public class CourseServiceBean implements CourseService{
@PersistenceContext(name="forPractise")
private EntityManager em;
@Resource
private SessionContext sessionCtx;
private Map<Integer, String> durationCode = null;
@EJB
private StudentService studentService;
........
......
...
@Override
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void mapCourseToStudents(Integer courseId,String studentIdList) throws Exception{
List<StudentEntity> studentEntityList = null;
TypedQuery<StudentEntity> studentQuery = null;
String query = "select s from StudentEntity s where s.studentId IN ("+studentIdList+")";
CourseEntity courseEntity = null;
try{
studentQuery = em.createQuery(query, StudentEntity.class);
studentEntityList = studentQuery.getResultList();
courseEntity = em.find(CourseEntity.class,courseId);
courseEntity.setStudentList(studentEntityList);
em.merge(courseEntity);
}catch(Exception e){
sessionCtx.setRollbackOnly();
throw e;
}
}
}
这是我的 persistence.xml 文件，
<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="forPractise" transaction-type="JTA">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<jta-data-source>jdbc/app</jta-data-source>
<class>com.entity.StudentEntity</class>
<class>com.entity.DeptEntity</class>
<class>com.entity.CourseEntity</class>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.DerbyDialect" />
<property name="hibernate.show_sql" value="true" />
<property name="hibernate.format_sql" value="true" />
</properties>
</persistence-unit>
</persistence>
等待回复....