Hibernate many-to-many cascading delete

Question

I have 3 tables in my database: Students, Courses and Students_Courses

Students can have multiple courses and courses can have multiple students. There is a many-to-many relationship between Students and Courses.

I have 3 cases for my project and courses added to my Courses table.

• (a) When I add a user, it gets saved fine,
• (b) When I add courses for the student, it creates new rows in User_Courses - again, expected behaviour.
• (c) When I am trying to delete the student, it is deleting the appropriate records in Students and Students_Courses, but it is also deleting Courses records which is not required. Even if I don't have any user in a course, I want the course to be there.

Below is my code for tables and annotate classes.

    CREATE TABLE `Students` (
    `StudentID` INT(11) NOT NULL AUTO_INCREMENT,
    `StudentName` VARCHAR(50) NOT NULL 
    PRIMARY KEY (`StudentID`)
)

CREATE TABLE `Courses` (
    `CourseID` INT(11) NOT NULL AUTO_INCREMENT,
    `CourseName` VARCHAR(50) NOT NULL 
    PRIMARY KEY (`CourseID`)
)

CREATE TABLE `Student_Courses` (
    `StudentId` INT(10) NOT NULL DEFAULT '0',
    `CourseID` INT(10) NOT NULL DEFAULT '0',
    PRIMARY KEY (`StudentId`, `CourseID`),
    INDEX `FK__courses` (`CourseID`),
    INDEX `StudentId` (`StudentId`),
    CONSTRAINT `FK__courses` FOREIGN KEY (`CourseID`) REFERENCES `courses` (`CourseID`) ON DELETE NO ACTION,
    CONSTRAINT `FK_students` FOREIGN KEY (`StudentId`) REFERENCES `students` (`StudentId`)
)

This is the Java code generated by Hibernate:

@Entity
@Table(name = "Students")
public class Students implements java.io.Serializable {

    private Integer StudentID;
     private String Students;
    private Set<Courses> Courseses = new HashSet<Courses>(0);

    public Students() {
    }


    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "StudentID", unique = true, nullable = false)
    public Integer getStudentID() {
        return this.StudentID;
    }

    public void setStudentID(Integer StudentID) {
        this.StudentID = StudentID;
    }

    @Column(name = "Students", nullable = false, length = 50)
    public String getCampaign() {
        return this.Students;
    }

    public void setCampaign(String Students) {
        this.Students = Students;
    }


 @ManyToMany(cascade = {CascadeType.ALL}, fetch = FetchType.LAZY)
    @JoinTable(name = "Student_Courses", joinColumns = {
        @JoinColumn(name = "StudentId", nullable = false, updatable = false)}, inverseJoinColumns = {
        @JoinColumn(name = "CourseID", nullable = false, updatable = false)})
    public Set<Courses> getCourseses() {
        return this.Courseses;
    }

     public void setCourseses(Set<Courses> Courseses) {
        this.Courseses = Courseses;
    }

    }


    @Entity
@Table(name = "Courses")
public class Courses implements java.io.Serializable {

  private Integer CourseID;
    private String CourseName;
     private Set<Students> Studentses = new HashSet<Students>(0);

    public Courses() {
    }

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "CourseID", unique = true, nullable = false)
    public Integer getCourseID() {
        return this.CourseID;
    }

    public void setCourseID(Integer CourseID) {
        this.CourseID = CourseID;
    }

     @Column(name = "CourseName", nullable = false, length = 100)
    public String getCourseName() {
        return this.CourseName;
    }

    public void setCourseName(String CourseName) {
        this.CourseName = CourseName;
    }

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "Courseses")
    public Set<Students> getStudentses() {
        return this.Studentses;
    }

    public void setStudentses(Set<Students> Studentses) {
        this.Studentses = Studentses;
    }

    }

How can I achieve what I have described? I could not find any reasonable documentation on the web.

Answer 1

I found the correct mapping (and tested that with JUnit with an extensive case) in a similar scenario. I don't think I am going to post testing code because it would take long time to adapt to this example. Anyway the key is to:

• Not use mappedBy attribute for the annotations, use join columns
• List the possible CascadeTypes excluding REMOVE

In OP's example

@ManyToMany(fetch = FetchType.LAZY,
        cascade =
        {
                CascadeType.DETACH,
                CascadeType.MERGE,
                CascadeType.REFRESH,
                CascadeType.PERSIST
        },
        targetEntity = Course.class)
@JoinTable(name = "XTB_STUDENTS_COURSES",
        inverseJoinColumns = @JoinColumn(name = "COURSE_ID",
                nullable = false,
                updatable = false),
        joinColumns = @JoinColumn(name = "STUDENT_ID",
                nullable = false,
                updatable = false),
        foreignKey = @ForeignKey(ConstraintMode.CONSTRAINT),
        inverseForeignKey = @ForeignKey(ConstraintMode.CONSTRAINT))
private final Set<Course> courses = new HashSet<>();

@ManyToMany(fetch = FetchType.LAZY,
        cascade =
        {
                CascadeType.DETACH,
                CascadeType.MERGE,
                CascadeType.REFRESH,
                CascadeType.PERSIST
        },
        targetEntity = Student.class)
@JoinTable(name = "XTB_STUDENTS_COURSES",
        joinColumns = @JoinColumn(name = "COURSE_ID",
                nullable = false,
                updatable = false),
        inverseJoinColumns = @JoinColumn(name = "STUDENT_ID",
                nullable = false,
                updatable = false),
        foreignKey = @ForeignKey(ConstraintMode.CONSTRAINT),
        inverseForeignKey = @ForeignKey(ConstraintMode.CONSTRAINT))
private final Set<Student> students = new HashSet<>();

Extensive JUnit testing verified that:

• I can add courses to students and vice versa flawlessly
• If I remove a course from a student, the course is not deleted
• Vice versa
• If I remove a student, all courses are detached but they are still persisted (to other students) in database
• Vice versa