foreign key, null fields and one-to-many

dagda1 · **Joined:** Fri May 05, 2006 11:54 am **Posts:** 51

Hi,

I have been using NHibernate for a while now and am perplexed by one-to-many relationships.

For example, if I have the following scrapbook class which contains a one-to-many bag of ScrapBookArticle objects.

Here are the mapping files:

Scrapbook

Code:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="waitrose.core.domain.Scrapbook,waitrose.core" table="Scrapbooks" lazy="true">
      <id name="Uid" column="Uid" type="System.Guid" unsaved-value="00000000-0000-0000-0000-000000000000">
         <generator class="guid"/>
      </id>
      <bag name="ScrapbookArticles" inverse="false" lazy="true" generic="true" cascade="all">
        <key column="ScrapbookUid"/>
        <one-to-many class ="waitrose.core.domain.ScrapBookArticle,waitrose.core"/>
      </bag>
   </class>
</hibernate-mapping>

ScrapBookArticle

Code:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="waitrose.core.domain.ScrapBookArticle,waitrose.core" table="ScrapbookArticles" lazy="true">
    <id name="Uid" column="Uid" type="System.Guid" unsaved-value="00000000-0000-0000-0000-000000000000">
      <generator class="guid"/>
    </id>
    <property column="ArticleUrl" type="String" name="ArticleUrl" length="2048" />
    <property column="DateAdded" type="DateTime" name="DateAdded" />
    <property column="Title" type="String" name="Title" length="512" />
    <property column="Description" type="String" name="Description" length="2048" />
  </class> 
</hibernate-mapping>

In the ScrapbookArticles table, I have a ScrapbookUid foreign key column that I must set to allow nulls. This is because NHibernate first does an insert and then an update on this table.

For may next project I will not have the luxury of being able to change the DB schema and am worried that I will not be able to use NHibernate.

Can anyone please demistify whether I can have my foreign-key as not null? I do not see how I could use NHibernate otherwise.

I did find the following article that seemed to allude otherwise:

http://forum.hibernate.org/viewtopic.php?t=957582&highlight=foriegn+foreign+key+nullable

This must be a requirement that other people would like.

Thanks in advance

Paul

himawan · **Joined:** Sat Aug 19, 2006 6:31 pm **Posts:** 139

Your situation is described here.

http://www.hibernate.org/hib_docs/v3/re ... hild-bidir

The key thing to note is that the many side of the one-many relationship should be the owner. This would be the side that has the foreign key. Your mapping file of ScrapBookArticles doesn't seem to have the foreign key.

On the other side, you'd set inverse=true. Anyway this is all described in more detail in the link above.

Hope it helps.