Hibernate Annotations – Bidirectional One-To-Many

I was trying to use Hibernate Annotations, which I think is great … I am happy with it… but I learnt some stuff which I know I will forget. So I am blogging it here, hopefully I will make sense and serve as a reminder for me when I need it. Maybe it might help someone else too… who knows…

Bidirectional One-To-Many
here I am trying to create a Bidirectional One-To-Many relationship between Question and Choice, a Question has many choices.

1. Setup entities
   
@Entity
@Table (name = "QUESTION")
public class Question implements Serializable
{

   @Id
   @GeneratedValue (strategy = GenerationType.AUTO)
   @Column (name = “QUESTION_ID”)
   private Long id;

   @Column (name = “TEXT”)
   private String text;
   ……………

   
@Entity
@Table (name = "CHOICE")
public class Choice implements Serializable
{

   @Id
   @GeneratedValue (strategy = GenerationType.AUTO)
   @Column (name = “CHOICE_ID”)
   private Long id;

   @Column (name = “TEXT”)
   private String text;
   ……………

2. Setup the ONE side on Question – I read ONE Question has MANY CHOICE(S).
   Question
   
@OneToMany (mappedBy="question")
private Set choices = new HashSet();


   mappedBy – means “I am not the owner side”, I am mapped by question from the other side of the relationship. It will also not create the database column which makes sense, I would expect a foreign key on the CHOICE table instead.

3. Setup the Many side on Choice – I read on Choice.java MANY Choice has ONE Question
@ManyToOne
@JoinColumn (name="QUESTION_ID")
private Question question;


   the @JoinColumn indicate the owning side of the relationship, it is responsible for updating the database column. It will create the QUESTION_ID column on the CHOICE table

4. Reversing the Relationship so that the owning side is the Question instead
   Question
   
@OneToMany
@JoinColumn (name = "QUESTION_ID")
private Set choices = new HashSet();


   Choice
   
@ManyToOne
@JoinColumn (name="QUESTION_ID", updatable = false, insertable = false)
private Question question;


5. Finally I need to use a List instead of Set – so I can preserve the order of the Choice(s).Question
   
@OneToMany (
cascade = {CascadeType.ALL},
fetch = FetchType.EAGER
)
@JoinColumn (name = "QUESTION_ID")
@org.hibernate.annotations.Cascade(
value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN
)
@org.hibernate.annotations.IndexColumn(name = "CHOICE_POSITION")
private List choices = new ArrayList();


   Choice
   
@ManyToOne
@JoinColumn (name="QUESTION_ID", nullable = false, updatable = false, insertable = false)
private Question question;


   the @org.hibernate.annotations.IndexColumn defines the colum CHOICE_POSITION that will be used to maintain the order of the list. Some how reversing the ownership is the only way to get the IndexColumn to work

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