JPA , many-to-many relation, deleting all previous relations and entering the new relations

Question

here i am trying out many-to-many relationship in JPA, I'v created tables "tblcourse" and "tblStudent", a student can register to many courses, ``` 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, …….. …. ); ``` The JPA representation of the above relation is as follows, this is the code for StudentEntity.java, ``` @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(…

Accepted Answer

I might be wrong but I think this is normal that when you're making your insertions, Hibernate firsts deletes all the records from the associated table.

That's the reason : when working with x-to-many association (basically, associations that are mapped through a Collection), Hibernate's persistence context will perfom the dirty checkings based on the Collection's identifier.

Let's take the mapCourseToStudents() method from your CourseServiceBean class :

```
    ...
    studentQuery = em.createQuery(query, StudentEntity.class);
    studentEntityList = studentQuery.getResultList();
    courseEntity = em.find(CourseEntity.class,courseId);
    courseEntity.setStudentList(studentEntityList); // replacing the previous Collection by the one you retrieved by querying the DB !!! 
    em.merge(courseEntity);
    ...
```

If you really want to avoid Hibernate executing the delete statement first, you should add/remove items to the Collection instead of assigning a new Collection and configure in the mapping data the operations that you want to cascade.

JPA , many-to-many relation, deleting all previous relations and entering the new relations

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