Annotation approach for http://www.hibernate.org/118.html#A

wei725 · **Joined:** Tue Feb 17, 2009 3:36 pm **Posts:** 78

Question:

Quote:

I have a many-to-many association between two tables, but the association table has some extra columns (apart from the foreign keys). What kind of mapping should I use?

from http://www.hibernate.org/118.html#A10

I have read through the reference documentation and can't find any reference for this subject. Can someone point me to a right direction?

Thanks.

littypreethkr · **Posted:** Fri Mar 06, 2009 5:41 am

The best thing to do is map the link table also to an entity and have MTO association b/w the main entities and link entity.

Code:

Like:
TABLE_A MTM TABLE_B through TABLE_AB

Then:
Class A mapped to TABLE_A
Class B mapped to TABLE_B
Class AB mapped to TABLE_AB

Now:
AB MTO A
AB MTO B

Now A and B can have a OTM collection mapping to AB

wei725 · **Joined:** Tue Feb 17, 2009 3:36 pm **Posts:** 78

littypreethkr wrote:

The best thing to do is map the link table also to an entity and have MTO association b/w the main entities and link entity.

Code:

Like:
TABLE_A MTM TABLE_B through TABLE_AB

Then:
Class A mapped to TABLE_A
Class B mapped to TABLE_B
Class AB mapped to TABLE_AB

Now:
AB MTO A
AB MTO B

Now A and B can have a OTM collection mapping to AB

Thanks for your input, but where is annotation? I have done the type of mapping with XML files before. Now, I want to try the annotation approach for the same case.

littypreethkr · **Posted:** Mon Mar 09, 2009 1:24 am

Code:

@Entity
@Table(name = "TABLE_A")
public class ClassA {

   long id;
   String name;
   Set<ClassAB> abs;
   
   @Id
   @GeneratedValue
   public long getId() {
      return id;
   }

   @Column(name = "A_NAME")
   public String getName() {
      return name;
   }

   @OneToMany(cascade = CascadeType.ALL)
   @JoinColumn(name = "A_ID", nullable = false)
   public Set<ClassAB> getAbs() {
      return abs;
   }

   //Setters
}

@Entity
@Table(name = "TABLE_B")
public class ClassB {

   long id;
   String value;
   Set<ClassAB> abs;
   
   @Id
   @GeneratedValue
   public long getId() {
      return id;
   }

   @Column(name = "B_VALUE")
   public String getValue() {
      return value;
   }
   
   @OneToMany(cascade = CascadeType.ALL)
   @JoinColumn(name = "B_ID", nullable = false)
   public Set<ClassAB> getAbs() {
      return abs;
   }

   //Setters
}

@Entity
@Table(name = "TABLE_AB")
public class ClassAB {

   long id;
   String extra;
   ClassA a;
   ClassB b;

   @Id
   @GeneratedValue
   public long getId() {
      return id;
   }

   @Column(name = "AB_EXTRA")
   public String getExtra() {
      return extra;
   }

   @ManyToOne
   @JoinColumn(name = "A_ID", nullable = false)
   public ClassA getA() {
      return a;
   }

   @ManyToOne
   @JoinColumn(name = "B_ID", nullable = false)
   public ClassB getB() {
      return b;
   }

   //Setters
}

wei725 · **Joined:** Tue Feb 17, 2009 3:36 pm **Posts:** 78

Thanks for your following up. I am wondering whether you understand the original question or not. It is not about entity, but a composite element.

littypreethkr · **Posted:** Mon Mar 09, 2009 3:06 am

I suppose this was your question.

wei725 wrote:

Question:
I have a many-to-many association between two tables, but the association table has some extra columns (apart from the foreign keys). What kind of mapping should I use?

But if you go to that link you specified you will also see this:

http://www.hibernate.org/118.html#A10 wrote:

An alternative approach is to simply map the association table as a normal entity class with two bidirectional one-to-many associations.

I have specified the mapping for this approach. I think this one is much straight forward and simpler than the other one of using composite elements.

wei725 · **Joined:** Tue Feb 17, 2009 3:36 pm **Posts:** 78

littypreethkr wrote:

I suppose this was your question.

wei725 wrote:

Question:
I have a many-to-many association between two tables, but the association table has some extra columns (apart from the foreign keys). What kind of mapping should I use?

But if you go to that link you specified you will also see this:

http://www.hibernate.org/118.html#A10 wrote:

An alternative approach is to simply map the association table as a normal entity class with two bidirectional one-to-many associations.

I have specified the mapping for this approach. I think this one is much straight forward and simpler than the other one of using composite elements.

Sorry, I should make it clear that I prefer the first approach which won't create another entity.

SIau_Tie · **Posted:** Fri Mar 13, 2009 1:27 am

wei725 wrote:

littypreethkr wrote:

I suppose this was your question.

wei725 wrote:

Question:
I have a many-to-many association between two tables, but the association table has some extra columns (apart from the foreign keys). What kind of mapping should I use?

But if you go to that link you specified you will also see this:

http://www.hibernate.org/118.html#A10 wrote:

An alternative approach is to simply map the association table as a normal entity class with two bidirectional one-to-many associations.

I have specified the mapping for this approach. I think this one is much straight forward and simpler than the other one of using composite elements.

Sorry, I should make it clear that I prefer the first approach which won't create another entity.

Hi wei725,

As I know, to add extra columns to many-to-many relationships, you should create another entity.

Please refer to this link
http://boris.kirzner.info/blog/archives/2008/07/19/hibernate-annotations-the-many-to-many-association-with-composite-key/

wei725 · **Joined:** Tue Feb 17, 2009 3:36 pm **Posts:** 78

SIau_Tie wrote:

Hi wei725,

As I know, to add extra columns to many-to-many relationships, you should create another entity.

Please refer to this link
http://boris.kirzner.info/blog/archives/2008/07/19/hibernate-annotations-the-many-to-many-association-with-composite-key/

Thanks for the link. I was thinking of an equivalent annotation statement which no Java code change is required. The Java code structure in the article/blog is quite different from the section of tips & tricks. To clarify my design approach, I list my business models as the followings:

Code:

@Entity
@Table(name = "group")
public class Group implements Serializable  {

       private Integer id;
   private Set<Membership> memberships;
        ...
}

public class Membership implements Serializable {

   public enum STATUS { DEDLINCED, PENDING, APPROVED };

   private Member           member;
   private Calendar    joinDate;
        ...
}

@Entity
@Table(name = "member")
public class Member implements Serializable  {

        private Integer id;
        ...
}

Where the Membership is a many-to-many relationship between groups and members without a composite-key. I know how it can be done with a XML mapping file, but not annotation.