bi-directional one-to-many

RF · **Joined:** Fri May 30, 2008 3:57 pm **Posts:** 26

NHibernate 1.2
.Net 3.5
SQL Server 2005

I can't seem to get a bi-directional one-to-many working properly, and I'm not sure why. Here is my configuration files with the irrelevant bits pulled out for brevity.

parent:

Code:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="Services.ProcedureDS.NH__Procedure, ProcedureDS"
         table="Procedures">
    
    <id name="id" column="id" type="Int32" >
      <generator class="identity" />
    </id>

    <set name="layouts" lazy="false" table="ProcedureLayouts" inverse="true" cascade="all">
      <key column="procedure_id"/>

      <one-to-many not-found="exception" class="Services.ProcedureDS.NH__ProcedureLayout, ProcedureDS"/>
    </set>
  </class>
</hibernate-mapping>

children:

Code:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="Services.ProcedureDS.NH__ProcedureLayout, ProcedureDS"
         table="ProcedureLayouts">
    
    <id name="id" column="id" type="Int32" >
      <generator class="identity" />
    </id>

    <property name="version" column="version" type="Byte" />
    <many-to-one name="parentProcedure" column="procedure_id" class="Services.ProcedureDS.NH__Procedure, ProcedureDS" />
    
  </class>
</hibernate-mapping>


I've been staring at section 6.8 of the documentation for the last couple of hours and I don't understand why this isn't working.

Can someone please point out what I'm doing wrong?

Thanks.

wolli · **Posted:** Thu Jul 03, 2008 3:06 am

The mapping looks good. What does not work ? Post the classes and the code where you try to save objects.

RF · **Joined:** Fri May 30, 2008 3:57 pm **Posts:** 26

Thank you for the reply Wolli, I tracked it down.

Apparently NHibernate requires you to explicitly set the parent of the child, which I didn't expect. Up until now I've been dealing with m:n relationships, and adding the child to the collection was enough, so the change in behavior was unexpected (for me atleast).

I'm still getting to know the quirks of NHibernate, but from what I've gathered, the inverse=true tells NHibernate that the child is going to manage the relationship (insofar as inserts/updates are concerned). Normally the parent would manage the relationship, but in the case of a non-null parent id in the child, you run into a chicken and egg problem, hence the need for the child to manage it.

In a m:n relationship you don't have this problem because the the relationship exists outside of the both the parent and child. ie, both can be persisted without the relationship getting in the way.

Please let me know if I'm still misunderstanding the issue's surrounding 1:n vs m:n collections.

wolli · **Posted:** Thu Jul 03, 2008 11:21 am

If you need more information on the Parent-Child handling, have a look here:

http://www.hibernate.org/hib_docs/nhibernate/1.2/reference/en/html/example-parentchild.html