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.
User_Courses
- again, expected behaviour.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.
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:
mappedBy
attribute for the annotations, use join columnsCascadeTypes
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: