I stumbled on a problem with merge method of EntityManager.
I have this class
@Entity
public class QuestionList {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String name;
@ManyToOne
private UserEntity user;
@OneToMany(cascade = CascadeType.ALL)
private List<QuestionUnit> units = new ArrayList<QuestionUnit>();
public List<QuestionUnit> getUnits() {
return units;
}
public void setUnits(List<QuestionUnit> units) {
this.units = units;
}
public Long getId() {
return id;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void addQuestionUnit(QuestionUnit QuestionUnit) {
this.units.add(QuestionUnit);
}
public void setUser(UserEntity user) {
this.user = user;
}
public UserEntity getUser() {
return this.user;
}
}
now, if I just modify one of the objects in units the merge is fine, but if I delete or add new object to the collection, the way that merge happens is that I am getting delete
from
QuestionList_QuestionUnit
where
QuestionList_id=? and then every row is added one by one insert
into
QuestionList_QuestionUnit
(QuestionList_id, units_id)
values
(?, ?)
I am wondering, is it possible to tell JPA/Hibernate to just add/delete the row associated with the object added/deleted? Or is it the way that merge just works with collections and in my case I need to write my own query for updating the database.
I am asking because this collection can be potentially pretty huge, and just deleting all rows and inserting almost all of them again doesn't seems like a good way to update the table.
EDIT: as requested:
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class QuestionUnit {
@Id
@GeneratedValue(strategy = GenerationType.TABLE)
private long id;
@OneToOne(cascade = CascadeType.ALL)
private AnswerUnit correctAnswer;
public QuestionUnit(AnswerUnit correctAnswer) {
this.correctAnswer = correctAnswer;
}
public void setCorrectAnswer(AnswerUnit correctAnswer) {
this.correctAnswer = correctAnswer;
}
public AnswerUnit getCorrectAnswer() {
return this.correctAnswer;
}
public long getId() {
return id;
}
public void setId(long id){
this.id=id;
}
public abstract Object getQuestionContent();
public abstract String getTitle();
}
there are 3 concrete classes in this hierarchy, for example
@Question("openQuestion")
@Entity
public class OpenQuestion extends QuestionUnit {
public void setQuestionContent(String questionContent) {
this.questionContent = questionContent;
}
private String questionContent;
public OpenQuestion() {
this(null, new OpenQuestionAnswer());
}
public OpenQuestion(String questionContent, AnswerUnit correctAnswer) {
super(correctAnswer);
this.questionContent = questionContent;
}
public String getQuestionContent() {
return this.questionContent;
}
public String getTitle() {
return questionContent;
}
}
@Transactional(propagation = Propagation.REQUIRED)
public void update(QuestionList questionList) {
entityManager.merge(questionList);
}
this method is used for all updates, because the list is in the detached state.
