一对多休眠保存级联问题

Question

I was working with Hibernate and OneToMany Relationship, i came across a weird behavior today , although i did it purposely but i was not expecting this behavior

College.java

@Entity
@Table(name="COLLEGE")
public class College implements Serializable{
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="COLLEGE_XID")
    private Long collegeId;

    @Column(name="COLLEGE_NAME")
    private String collegeName;

    @OneToMany(mappedBy="college",cascade = CascadeType.ALL) 
    private List<Student> studentList = new ArrayList<Student>();
    //... with get and setters
}

Student.java

@Entity
@Table(name = "STUDENT")
public class Student implements Serializable    {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="STUDENT_XID")
    private Long studentId;

    @Column(name="STUDENT_NAME")
    private String studentName;

    @ManyToOne
    @JoinColumn(name="COLLEGE_XID",nullable=false)
    private College college;
    //... with get and setters
}

Application.java

    College college1 = new BdCollege();
    college1.setCollegeName("C_1");

    College college2 = new BdCollege();
    college2.setCollegeName("C_2");

    Student student1 = new BdStudent();
    student1.setStudentName("S_1_C_2");

    CollegeDAO collegeDAO = new  CollegeDAO();

    // setting student 1 Parent to College 1        
    student1.setCollege(college1);   // <---- Here the Issue  ... student 1 as college 1 child

    // BUT assigning Student 1 as Child of Collge 2 in List 
    // i did it purposely to test the behaviour
    college2.getStudentList().add(student1);  // <---- Here the Issue ... but assigning it in the list of College 2 ... ws expecting Exception 

     // Saved College 1 & 2
     collegeDAO.save(college1);
     collegeDAO.save(college2);

      // Saved Successfully BUT

RESULT COLLEGE Table

 COLLEGE_XID     COLLEGE_NAME    
 --------------  --------------- 
 1               C_1             
 2               C_2 

STUDENT Table

 STUDENT_XID     STUDENT_NAME     COLLEGE_XID    
 --------------  ---------------  -------------- 
 1               S_1_C_2          1   

I was expecting some exception as assignment from child and parents mis-matched in college - student example and another thing that stucked me were the query generated by hibernate

Hibernate: insert into COLLEGE (COLLEGE_NAME) values (?)
Hibernate: insert into COLLEGE (COLLEGE_NAME) values (?)
Hibernate: insert into STUDENT (COLLEGE_XID, STUDENT_NAME) values (?, ?)

ie it save College 1 first, Then College 2 Second and Saving Student as Child of College 2 but in the result it appears as child of College 1

Answer 1

Its because you added mappedBy attribute and made student as owner of association. Since Student entity is the owner, it will maintain the relationship and hibernate will ignore changes on College side.