ManyToMany & fields of association table

[Musab Al-Rawi]

Hello,

given that you have tables A and B that have a Many-to-Many relationship implemented using table C, where Table C has some fields related to the relationship (for example start_date, due_date whatever).

How can you access fields of the association table.

I know that we need to use the ManyToMany with mappedBy and JoinColumn but in class A i will have a Set<B> and in B I will Set<A>, where does C come and how can I access those fields that are related to the association (such as start_date).

thanks in advance.

[Musab Al-Rawi]

Here is an example to explain my questions:
let's say there is entity FOLDER that has a many to many relationship with GROUP. I could do the design as follows:

+------+        +------------+     +-----+
|FOLDER|--------|FOLDER_GROUP|-------|GROUP|
+------+ 1    n +------------+ n    1+-----+
  |                |    |  |           |
  |                |    |  |           |
  |                |    |  |           |
(F_ID)            (G_ID)| (INFO)      (G_ID)
                        |
                      (F_ID)
 

Where and How can I represent INFO?
Is the following a better design (of courseDUMMY entity won't be named DUMMY!)?

 
+-----+
|GROUP|----(G_ID)
+-----+
  1|
   |
                   |
                  n|
+------+        +------------+     +-----+
|FOLDER|--------|FOLDER_GROUP|-------|DUMMY|
+------+ 1    n +------------+ 1    1+-----+
  |                |    |  |           |   |
  |                |    |  |           |   |
  |                |    |  |           |   |
(F_ID)            (G_ID)| (D_ID)     (D_ID)|
                        |                (INFO)
                      (F_ID)
 

Personally I want to go for the first design but I don't know how to represent INFO field.
In that case I define Folder and Group entities as follows:
In Folder
folder

@ManyToMany
       @JoinTable(
               schema="test", name="folder_group",
               joinColumns=
                       @JoinColumn(name="folder_id", referencedColumnName="id"),
               inverseJoinColumns=
                       @JoinColumn(name="group_id", referencedColumnName="id")
       )
       private Set<Group> folderPriv;

In Group

@ManyToMany (mappedBy="folderPriv")
private Set<Folder> folderPriv;

Again how to access INFO column located in folder_group table?

Your help is greatly appreciated.
[ October 28, 2008: Message edited by: Musab Al-Rawi ]

[Chaminda Amarasinghe]

Hi,

Apparently very common situation, I also thought when I was reading Many-Many relationships. But I couldn't find a way..





I will try again

Thanks

[Chaminda Amarasinghe]

Hi Rawi,

where are you going to put start_date? class A or B? I think you cant put it in any one of them. that does not belong to A or B directly.

I think in this type of situation we need another relationship and entity class called C. We have to devide this m-m relation ship to two 1-m and m-1 relationships

In this example

+------+ +------------+ +-----+
|FOLDER|--------|FOLDER_GROUP|-------|GROUP|
+------+ 1 n +------------+ n 1+-----+
| | | | |
| | | | |
| | | | |
(F_ID) (G_ID)| (INFO) (G_ID)
|
(F_ID)

One Folder has Many Or FolderGroupInfor (or FOLDER_GROUP ) and Each FolderGroupInfor has a Group

On other side,

One Group has Many FolderGroupInforand A FolderGroupInfor has a Group

/**
 *
 * @author Chaminda Amarasinghe <[e-mail]>
 */
 
class Folder {
 
    @OneToMany
    List<FolderGroupInfor> folderInfors ;
    
}
 
class FolderGroupInfor {
    
    @ManyToOne
    Folder folder;
    @ManyToOne
    Group group;
    
    // Other Infonation 
    String info;
}
 
class Group {
    
    @OneToMany
    List<FolderGroupInfor> folderInfors ; 
    
}

What about others thought ?

Thanks
Cham

[Musab Al-Rawi]

Thanks Cham,

I thought about that, but if that's the only solution (I don't know if it is or not that's why I am asking Ranchers) then probably ManyToMany works association table consists of PK of Entity tables (in other words not complete solution).

Experienced People, input please