I've gone through the FAQ's, read the first 3 chapters of King/Bauer's book and browsed Wrox's Hibernate book but I'm still a little unsure about many-to-many relationships. According to King and Bauer, its better to steer clear of these if possible and instead "use two one-to-many associations for either side....[and use] an intermediate association class."
(page 225). In that spirit, Ive proceeded. To take a simple example from my app, I've got a many to many relationship between users and permissions (a user can have many permissions and a permission can be assigned to many users). Following King/Bauer I built an intermediate class called PermissionUser and can persist permissions, users, and the relationship between these objects. However, to assign a permission to a user and persist the user so that its permissions are also saved, I'm finding that I cant just add the permission to the user and save it. Instead I also have to:

1)instantiate an intermediate class (permissionuser)
2)assign the intermediate class to user *and* permission
3)save the the user *and* the permission class

I'm enclosing the relevant code below:

1)PermissionUser permissionuser = new PermissionUser();
2)permissionuser.setPermission(permission);
3)permissionuser.setUser(user);
4)user.addPermissionuser(permissionuser);
5)permission.addPermissionuser(permissionuser);
6)aSession.saveOrUpdate(user);
7)aSession.saveOrUpdate(permission);
8)aSession.flush();


Is there an easier way? This approach seems to have a bit too much coding overhead....ideally I'd like to just be able to assign the permissionuser object to the user object and save the user object. I was hoping ORM would spare me the trouble of having to perform the same operations on the permission object (e.g. steps 5 and 7).

(Actually in an ideal world I'd like to just assign the permission object to the the user object and get rid of the intermediate "association class"-- but that doesn't seem practical in light of King/Bauer's recommends on page 225).

Cheers,

Luke Fernandez
Weber State University



Hibernate version:2.1.4

Mapping documents:


<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="gen.Permission"
table="PERMISSION"
>

<!-- Common id property. -->
<id name="id"
type="long"
column="PERMISSION_ID"
unsaved-value="null">
<generator class="native"/>
</id>

<property name="name" />


<set
name="permissionusers"
inverse="true"
cascade="all-delete-orphan">
<key column="PERMISSION_ID"/>
<one-to-many class="gen.PermissionUser"/>
</set>

</class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>

<class name="gen.PermissionUser"
table="PERMISSION_USER"
>

<!-- Common id property. -->
<id name="id"
type="long"
column="PERMISSIONUSER_ID"
unsaved-value="null">
<generator class="native"/>
</id>

<many-to-one
name="permission"
class="gen.Permission"
column="PERMISSION_ID"
update="false"
not-null="true"
outer-join="false"
foreign-key="FK1_PERMISSION_ID"/>


<many-to-one
name="user"
class="gen.User"
column="USER_ID"
update="false"
not-null="true"
outer-join="false"
foreign-key="FK1_USER_ID"/>




</class>

</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="gen.User"
table="USER"
>

<!-- Common id property. -->
<id name="id"
type="long"
column="USER_ID"
unsaved-value="null">
<generator class="native"/>
</id>

<property name="firstname" />
<property name="lastname" />
<property name="email" />
<property name="login" />
<property name="password" />

<set
name="teamusers"
inverse="true"
cascade="all-delete-orphan">
<key column="USER_ID"/>
<one-to-many class="gen.TeamUser"/>
</set>

<set
name="permissionusers"
inverse="true"
cascade="all-delete-orphan">
<key column="USER_ID"/>
<one-to-many class="gen.PermissionUser"/>
</set>

</class>
</hibernate-mapping>





Name and version of the database you are using: MySQL