Many to Many & Extra Attributes

anortje · **Joined:** Fri Oct 31, 2003 11:51 am **Posts:** 3

I have just started using Hibernate and have successfully setup a many to many relationship. I have one small problem though...

I have three tables, user, role and user_role. user_role joins user and role in the many to many relationship. In my User class the getRoles() method returns a Set of roles assigned the given User. However the user_role table has a column which indicates which is the default role. How do I get this attribute, which is in the user_role table, into the Role class (which maps to the role table).

TIA

nvolpini · **Posted:** Tue Nov 04, 2003 12:16 pm

You can create a class for the user_role table ...

Code:

public class UserRole {
   User user;
   Role role;
   boolean default;
   .....
}

<class name="User" table="user">
   ....
   ....
   <set name="roles" table="user_role" lazy="true">
      <key>
      <column name="user_id" not-null="true"/>
   </key>
   <one-to-many class="UserRole"/>
    </set>
    
</class>


<class name="UserRole" table="user_role">

    <composite-id>
       <key-many-to-one name="user" class="User" column="user_id"/>
   <key-many-to-one name="role" class="Role" column="role_id"/>
   </composite-id>

        <property name="default"/>
     
    
</class>

I think that exists another way to implement but I use this.

stolley · **Posted:** Tue Nov 04, 2003 8:18 pm

We actually use explicit modelling of many-many relationships as a general approach in our application.
We usually have a minimum of at least four extra auditing attributes (userCreated, userUpdated, etc) and often more - a type mapping to a reference table (making the whole thing a ternary mapping), a primary role indicator like yours; and sometimes other stuff.

We also always map our many-many "relationship" classes with a surrogate key (completely seperate from the foreign keys the table exists to model) as well!

I know it sounds horrible, and I was really against it at first - because I thought that it was wasting half the potential of an OR/Mapper - ie. transaparent mapping of collections. I thought it would be quite tedious to implement, in terms of having to write the relationship classes.

But it turns out not to have been much of an issue so far. It makes it easy to teach non-OR/Mapping people how to map many-many relationships, and it kind of makes it explicit for them so they can see what's going on instead of trying to imagine (and make wrong assumptions about) what Hibernate is doing.

gavin · **Posted:** Tue Nov 04, 2003 8:23 pm

We used the same approach on our last project. Its not horrible at all - in fact I think it is probably a best practice.

But in 2.1, composite elements do offer a really great alternative solution to the problem above.

max · **Posted:** Wed Nov 05, 2003 4:18 am

gavin - please put an example of this (maybe even in context of the concrete model)....that would be great to have in the same context as this thread....