many-to-many mapping to same class

dbrady · **Joined:** Thu Aug 18, 2005 5:55 am **Posts:** 2

Hi guys,

this is my first post on the forum so please forgive me if my question has already been answered. I've searched the forums, online doco etc, and still can't find a solution for what I'm trying to do.

I'm using hibernate 2.1, with mySql 4.01 and java1.4.2_08

I want to create a many-to-many mapping from a class to itself.

For example, I have a User class who submits documents for review. A User can both be a reviewer and a reviewee. So I would have something like the following:

Code:

public class User
{
  Set reviewers;
  Set reviewees;

  /* when I add a reviewer, I must become a reviewee for this user */
  public void addReviewer(User rr)
 {
    rr.addReviewee(this);
    reviewers.add(rr);
 }

 /* when I add a reviewee, I must become a reviewer for them */
 public void addReviewee(User re)
 {
    re.addReviewer(this);
    reviewees.add(re);
 }

 /* return a set of users who reviewed my document */
 Set getReviewers() { return reviewers; }

 /* get a list of users who's documents I reviewed */
 Set getReviewees() { return reviewees; }
}

I can't get mapping correct:

Code:

 <class name="user" class="User" table="USER">
     <set role="reviewers"  table="USER_REVIEWS">
         <key column="reviewer_id"/>
        <many-to-many column="reviewee_id" class="User"/>
    </set>
    <set role="reviewees" table="USER_REVIEWS">
        <key column="reviewee_id"/>
        <many-to-many columm="reviewer_id" class="User"/>
    </set>
 </class>

I'd expect to have two table in my database, USER table and a USER_REVIEWS table. The second table I'd expect to contain two columns, a reviewer_id and a reviewee_id that are both foreign keys back to the user table, with both columns making up the primary key.

I'd appreciate any help, even if its just a RTFM pointed at the right place ;-)
Thanks.
dbrady.

aabeling · **Posted:** Fri Aug 19, 2005 2:49 am

What problem do you have with your mapping? Can you provide any error messages? When does the error occur?

I don't know if there is something like a 'role' attribute to the set element but AFAIK a 'name' attribute is mandatory, so perhaps it works with

Code:

And there seems to be a spelling mistake in

Code:

<many-to-many columm="reviewer_id" class="User"/>

Generally the mapping looks ok.

dbrady · **Joined:** Thu Aug 18, 2005 5:55 am **Posts:** 2

I managed t get this working, so thought I'd post the code snippet for the benefit of others trying to do something similiar:

Code:

<class name="mypackage.User" table="USERS">
   <id name="id" column="userID_pk" type="int" unsaved-value="0">
         <meta attribute="scope-set">protected</meta>
         <generator class="increment" />
   </id>

   <set name="reviewers" table="USER_REVIEWERS" lazy="false">
      <meta attribute="field-description">Set of Reviewers for this User</meta>
      <key column="REVIEWEE_ID"/>
      <many-to-many column="REVIEWER_ID" class="mypackage.User" />
   </set>
         
   <set name="reviewees" table="USER_REVIEWERS" lazy="false">
      <key column="REVIEWER_ID"/>
      <many-to-many column="REVIEWEE_ID" class="mypackage.User"/>
   </set>

and in the User.java file you must be careful to avoid an infinite loop when adding reviewer/reviewee. The following works:

Code:

    public Set getReviewers()
    {
        return this.reviewers;
    }

    public void setReviewers(Set reviewers)
    {
        this.reviewers = reviewers;
    }

    public Set getReviewees()
    {
        return this.reviewees;
    }

    public void setReviewees(Set reviewees)
    {
        this.reviewees = reviewees;
    }

    public void addReviewer(User reviewer)
    {
        if (reviewers == null)
        {
            reviewers = new HashSet();
        }

        if (!reviewers.contains(reviewer))
        {
            reviewers.add(reviewer);
            reviewer.addReviewee(this);
        }
    }

    public void addReviewee(User reviewee)
    {

        if (reviewees == null)
        {
            reviewees = new HashSet();
        }

        if (!reviewees.contains(reviewee))
        {
            reviewees.add(reviewee);
            reviewee.addReviewer(this);
        }
    }

This results in a user_reviewer table with two columns reviewee_id and reviewer_id which combine to form the primary key and each refers back to the user table, one for the user who is the reviewer and the other who is the reviewee.

Cheers,
dbrady.