How to map an optional foreign key

besthofmed · **Joined:** Wed Jan 23, 2008 11:03 am **Posts:** 2

Hi everyone,

I have two tables parent and child. Child of course contains a foreign key so it can references his parent. But according to my business model, a Child can be orphan (it has no parent) so its foreign key should be null or zero in that case.

The problem arises from the fact that the generated DDL by hibernate (using Oracle dialect) creates a foreign key constraint (Child references Parent). So every time I try to add a new orphan child (without linking it to a parent), it throws an SQLException (violation of constraint).

I have added not-null="false" to both end of the association (so foreign key could and can be null) but nothing changed. I have used many-to-one from the child's side and one-to-many from the parent's side with inverse=true.

Please can anyone tell me how to make this association optional so I can add orphan children (I do not want to use dirty techniques and just drop the constraint from Oracle, I want hibernate to be aware of this too).

if you need more details I will be happy to add it.

thanks in advance

mimi_tan1985 · **Posted:** Wed Jan 30, 2008 12:08 am

can you paste the hbm.cfg.xml here?

X10 · **Joined:** Mon Feb 11, 2008 9:41 am **Posts:** 2

Hi!

I have the same problem. How do I model a optional foreign key????

Here are my mapping files:

Code:

<hibernate-mapping>

    <class
        name="metainf.anwendungskern.metainfoManagement.db.AttributeMetainformation"
        table="ET_ATMI_ATTRIBUTE_META_INFORMATION">

        <composite-id name="primaryKey" class="metainf.anwendungskern.metainfoManagement.db.AttributeMetainformationPK">
           <key-many-to-one name="attribute" class="metainf.anwendungskern.attributeManagement.db.Attribute" column="ATTR_KEY"/>
           <key-many-to-one name="useCase" class="metainf.anwendungskern.usecaseManagement.db.UseCase" column="USE_CASE_CONFIG"/>
      </composite-id>

   <many-to-one
            name="attributeGroup"
            cascade="none"
            column="ATGR_KEY"
            foreign-key="FK_ATTRIBUTEGROUP"
            lazy="false"
            fetch="join"/>
        
<!-- This one should be optional -->
        <many-to-one
            name="attributeClass"
            cascade="none"
            lazy="false"
            column="ATCL_KEY"
            foreign-key="FK_ATTRIBUTECLASS"
            fetch="join"/>

        ... something other...    

    </class>


</hibernate-mapping>

Code:

<hibernate-mapping>

    <class
        name="metainf.anwendungskern.metainfoManagement.db.AttributeClass"
        table="ET_ATCL_ATTRIBUTE_CLASS"
        batch-size="5">

        <id name="attributeClassKey" type="metainf.db.TrimmedString">
            <column name="ATCL_KEY"/>
        </id>
        
    </class>

</hibernate-mapping>

AttributeClass should be optional. Have anyone a solution?

Thanks in advance.

regards,
Pierre

cricdigs · **Joined:** Thu Feb 21, 2008 1:37 pm **Posts:** 3

I had a similar need and after doing a lot of googling found this undocumented feature in hibernate. You can use the foreign key name as "none" to remove the foreign key constraint. Hope this helps and I hope they make this a documented feature in the future releases.

cricdigs · **Joined:** Thu Feb 21, 2008 1:37 pm **Posts:** 3

I found this info on this very informative blog. Hope it will help others too:

http://blog.xebia.com/2007/02/05/let-hi ... our-world/