Many-To-Many with additional columns

pjhepworth · **Joined:** Tue Mar 20, 2007 7:51 am **Posts:** 3

Hi,

I was wondering if what I'm trying to achieve is possible, and if so... how?

Basically I have a many-to-many mapping between two objects, relating to three tables (one for each of the classes, and a third to link the two together).

However, the third linking table needs to have additional values (namely last_updated_by and last_updated_date).
i.e. TBL_A_TO_B needs 2 more columns in addition to the 2 keys.

How would I go about modelling these extra columns so that they can be updated??

Any help much appreciated!!
Paul

Code:

<class name="my.package.ClassA" table="TBL_A">
    <id name="aId" type="big_decimal">
        <column name="A_ID" precision="22" scale="0" />
        <generator class="assigned" />
    </id>
    <property...
    ...property>
    <set name="bs" table="TBL_A_TO_B">
        <key>
            <column name="A_ID" precision="22" scale="0" not-null="true" />
        </key>
        <many-to-many entity-name="my.package.ClassB">
            <column name="B_ID" precision="22" scale="0" not-null="true" />
        </many-to-many>
    </set>
</class>

batmat · **Posted:** Thu Mar 29, 2007 6:06 am

Well, it's simple : when you've got a central entity with properties, you don't define a many-to-many anymore, but a collection (see which one you want to use) of this central entity in the two ends of the relation.

From the central entity, map a <many-to-one /> relationship to both entities and you're done.

See the Java Persistence with Hibernate book if you need more details (p.303).

pjhepworth · **Joined:** Tue Mar 20, 2007 7:51 am **Posts:** 3

OK, I see what you're saying, but if I'm understanding correctly, then I will end up with methods to get the links (central objects) rather than the other entity.

So rather than having A.getBs(), I will have A.getABLinks() (returning a new class with the setLastUpdatedDate and setLastUpdatedBy methods) and then need to call getB on each of the link objects.

Is that the way it needs to be done?
Cheers

batmat · **Posted:** Thu Mar 29, 2007 6:34 am

pjhepworth wrote:

So rather than having A.getBs(), I will have A.getABLinks() (returning a new class with the setLastUpdatedDate and setLastUpdatedBy methods) and then need to call getB on each of the link objects.

Is that the way it needs to be done?
Cheers

Yes, exactly. Apart from the fact that you can decide to expose or not publicly visible methods to set the central additional property (lastUpdatedDate here). In fact, you could even map it with access="field" and write the code inside this object instead of managing it externally if you prefer.