why i get null at relationship field?

Question

Why if i run this code:

Student st = new Student();
st.setFirstName("First");
st.setLastName("Last");
st.setIndexNr("11");
st.setStudentPK(new StudentPK(0, user.getIdUser()));
studentFacade.create(st);

Mail m = new Mail();
m.setContent("con");
m.setRecipient("rec");
m.setIdMail(0);
mailFacade.create(m);

List<Mail> l = new ArrayList<Mail>();
l.add(m);
st.setMailList(l);
studentFacade.edit(st);     // st have mailList property set to l

stud=studentFacade.findByIndex("11"); //after edit and get student he has mailList equal null

Why after persist and edit object i get null at property for OneToMany relationship?

---

In database MySql i have STUDENT table and MAIL table:

CREATE  TABLE IF NOT EXISTS `STUDENT` (
  `id_student` MEDIUMINT UNSIGNED NOT NULL AUTO_INCREMENT ,
  `first_name` VARCHAR(65) NULL ,
  `last_name` VARCHAR(65) NULL ,
  `index_nr` VARCHAR(45) NULL 
)
ENGINE = InnoDB

CREATE  TABLE IF NOT EXISTS `MAIL` (
 `id_mail` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
 `recipient` TEXT NULL ,
 `content` TEXT NULL ,
 `sender_student` MEDIUMINT UNSIGNED NULL ,
  PRIMARY KEY (`id_mail`) ,
  INDEX `fk_STUDENT_id_idx` (`sender_student` ASC) ,
  CONSTRAINT `fk_STUDENT`
 FOREIGN KEY (`sender_student` )
 REFERENCES `jkitaj`.`STUDENT` (`id_student` )
 ON DELETE CASCADE
 ON UPDATE CASCADE
)
ENGINE = InnoDB

From database i generate entity in netbeans:

@Entity
@Table(name = "STUDENT")
public class Student implements Serializable {
 private static final long serialVersionUID = 1L;
 @EmbeddedId
 protected StudentPK studentPK;
 @Size(max = 65)
 @Column(name = "first_name")
 private String firstName;
 @Size(max = 65)
 @Column(name = "last_name")
 private String lastName;
 @Size(max = 45)
 @Column(name = "index_nr")
 private String indexNr;
 @OneToMany(mappedBy = "senderStudent",fetch=FetchType.EAGER)
 private List<Mail> mailList;

 //getters, setters ...

}

public class Mail implements Serializable {
 private static final long serialVersionUID = 1L;
 @Id
 @GeneratedValue
 @Basic(optional = false)
 @Column(name = " id_mail")
 private Integer idMail;
 @Lob
 @Size(max = 65535)
 @Column(name = "recipient")
 private String recipient; 
 @Lob
 @Size(max = 65535)
 @Column(name = "content")
 private String content;
 @JoinColumn(name = "sender_student", referencedColumnName = "id_student")
 @ManyToOne
 private Student senderStudent;

 //getters, setters...
}

---

EDIT:

I think i forget about fetch in @OneToMany annotation of Student entity. But when i set in to fetch=FetchType.LAZY i again get null after edit and and get edited object from database. When set fetch=FetchType.EAGER mailList field isn't null. Why ?

Answer 1

Problem is at OpenJPA, when i use fetch=FetchType.LAZY at some property of entity. The problem was with life cycle of enties. All enties must be re-attached to the persistence context.

Same problem is here: OpenJPA - lazy fetching does not work and here: What's the lazy strategy and how does it work?