Spring MVC can not delete a parent row? - mysql

Question

I started to learn spring mvc. I was doing some practises and i encountered a logical error. In my example I have 2 entity class in spring mvc. Down there my code school_ıd is the primary key. When I try to delete a school from school list I gave an error like this
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails
Entity school:
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name="school_id")
private int schoolId;
#Column(name="school_name")
private String schoolName;
public int getSchoolId() {
return schoolId;
}
public void setSchoolId(int schoolId) {
this.schoolId = schoolId;
}
public String getSchoolName() {
return schoolName;
}
public void setSchoolName(String schoolName) {
this.schoolName = schoolName;
}
Entity Student
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name="id")
private int id;
#Column(name="first_name")
private String firstName;
#Column(name="last_name")
private String lastName;
#Column(name="email")
private String email;
#OneToOne
#JoinColumn(name="school_id")
private School school;
public School getSchool() {
return school;
}
public void setSchool(School school) {
this.school = school;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
School Controller delete method:
#GetMapping("/deleteSchool")
public String deleteSchool(#RequestParam("schoolID") int
theSchoolId) {
schoolService.deleteSchool(theSchoolId);
return "redirect:/school/list";
}
Delete Method SchoolDAOImpl:
#Override
public void deleteSchool(int theSchoolId) {
Session currentSession=sessionFactory.getCurrentSession();
Query theQuery=currentSession.createQuery("delete from School where id=:schoolID");
theQuery.setParameter("schoolID", theSchoolId);
theQuery.executeUpdate();
}
Actually i now the problem is i attemp to delete a school, theSchool has at least 1 student because of that i cannot delete a school. For this first i need to delete child in this example child is student after delete parent(school).
But i think my scenario does not avaliable for this. Please help me what should ı do?

Answer 1

You can update Student first:
#Override
public void deleteSchool(int theSchoolId) {
Session currentSession=sessionFactory.getCurrentSession();
Query updateStudentQuery=currentSession.createQuery("Update Student s SET s.school = null WHERE s.school.schoolId = :schoolId");
updateStudentQuery.setParameter("schoolID", theSchoolId);
updateStudentQuery.executeUpdate();
Query theQuery=currentSession.createQuery("delete from School where id=:schoolID");
theQuery.setParameter("schoolID", theSchoolId);
theQuery.executeUpdate();
}
or update using Stundent's DAO and then exec the school's delete query.
I couldnt test so i dont know if it works the "WHERE s.school.schoolId = :schoolId" or you have to make a where with a subquery. If doesn't works, try with a native query:
Query updateStudentQuery=currentSession.createNativeQuery("sql here");
Instead of
Query updateStudentQuery=currentSession.createQuery("Update Student s SET s.school = null WHERE s.school.schoolId = :schoolId");
Also, if you want to make the relation bidirectional, add this to School Entity:
#OneToOne(mappedBy = "school")
private Student student;
By the way, i guess that the relation is #ManyToOne and not #OneToOne, isn't it?

Answer 2

You have to delete the integrity constraint of whatever the DB you are using or you should first delete the child and then parent or you can look the cascade rule of that in hibernate https://thoughts-on-java.org/hibernate-tips-how-to-delete-child-entities/