How to delete many-to-many association?

nvolpini · **Posted:** Tue Aug 26, 2003 4:10 pm

Situation:
I have an User and a Group. The users can have some groups, therefore I have an n-n relationship.
Tables:
user, group and user_group.

I am adding a group to the user:

group = new Group(1);
user.getGroups().add(group) # Just add a tuple in user_group. OK
session.update(user);
session.flush();

How to remove it ? I tried this but didn't work:

group = (Group) session.load(Group.class, id);
user.getGroups().remove(group);
session.update(user);
session.flush();

What's the solution?

bsiggelkow · **Posted:** Tue Aug 26, 2003 4:18 pm

I think you need to set

Code:

cascade=all

or

Code:

cascade=all-delete-orphan

.

dbenoff · **Joined:** Tue Aug 26, 2003 4:19 pm **Posts:** 42

Try group.getUsers.remove(user);
Also, the update is not needed, flush will save the changes.

nvolpini · **Posted:** Tue Aug 26, 2003 8:42 pm

Changing to

Code:

cascade="all"

or

Code:

cascade="all-delete-orphan"

or

Code:

group.getUsers().remove(user);

I get

net.sf.hibernate.HibernateException: Another object was associated with this id (the object with the given id was already loaded)

Because the group was "loaded", I think !!

If I instantiate a group without load doesn't work because the comparison doesn't occur only on id. Correct ?

Code:

Group group = new Group(id);
model.getPrivileges().remove(group);  #Does not work

Some idea ???

gavin · **Posted:** Tue Aug 26, 2003 9:14 pm

The contract for java.util.Set requires that equals()/hashCode() be implemented by classes that are contained in sets.

nvolpini · **Posted:** Tue Aug 26, 2003 9:33 pm

I need to implement equals() and hashCode() in the Group class ? Why? To implement a comparison just using the id of the class without load the object?

[/code]

steve · **Posted:** Tue Aug 26, 2003 9:40 pm

You really might want to check out the documentation on the java.util.Set interface. A Set, unlike a list, will not contain duplicate entries as determined by use of the hashCode() method.

When you try to remove a particular object from a Set, the hashCode() of the object object passed to the Set.remove() method is used to remove the appropriate object. Thus if you don't have hasCode() overriden properly, Set.remove() will probably have no effect.

And you need to override equals() because that should be done every time you override hashCode().

nvolpini · **Posted:** Tue Aug 26, 2003 10:55 pm

I had sucess implementing equals() and hashCode(). :-)

As I never made this before, I haven't sure if this is the best way to implement it.

Code:

public class Group {
int id;   #is the primary key
.....
public boolean equals(Object o) {
        if (o == this)
            return true;
        
        if (!(o instanceof Group))
       return false;
            
        Group g = (Group) o;
        
        if (g.id == this.id)
            return true;
        
        return false;
    }
    
    public int hashCode() {
        return this.id;
    }

Perhaps exists a better solution ? What you think?

leah · **Joined:** Wed Aug 27, 2003 1:47 pm **Posts:** 2

nvolpini wrote:

I had sucess implementing equals() and hashCode(). :-)

As I never made this before, I haven't sure if this is the best way to implement it.

Perhaps exists a better solution ? What you think?

I use Jakarta Commons Lang package

Code:

import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
.....
   public boolean equals(Object o) {
       if (!(o instanceof Child)) {
         return false;
       }
      Child rhs = (Child) o;
       return new EqualsBuilder()
                  .append(id, rhs.id)
                  .isEquals();
    }

   public int hashCode() {
     return new HashCodeBuilder(17, 37).   
      append(id).
      toHashCode();
   }