HowTo/Best Practices with Inheritance and Relations

mgaerber · **Posted:** Mon Jan 30, 2006 3:17 pm

Hi,

assuming I want to have a base class (BaseContent) in a CMS from wich I want to derive all other content classes (Article, ForumPost, Whatever) AND want to maintain relations using this base class, for example to categories and such.

How would I implement this best using NHibernate?

Thanks for any help!

DeanOC · **Posted:** Mon Jan 30, 2006 6:00 pm

This sounds similar to our class hierarchy, where we have a base class and other classes extend that base class. We have mapped our classes using sub-classes, e.g.

Code:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
  <class name="Domain.Model.Definition, Domain.Model" table="Definition" discriminator-value="Definition">
    <meta attribute="scope-class" inherit="false">public abstract</meta>
    <id name="Id" type="Persistence.NullGuidUserType, Persistence" column="Id">
      <generator class="assigned" />
    </id>
    <discriminator column="DEFINITION_TYPE" type="string" />
    <version name="Version" column="Version" type="Int32" unsaved-value="-1" />
   <subclass name="Domain.Model.ComputedAttributeDefinition, Domain.Model">
      <property name="AttributeDefinitionId" type="Persistence.NullGuidUserType, Persistence" column="AttributeDefinitionId"/>
   </subclass>
   <subclass name ="Domain.Model.ItemAttributeDefinition, Domain.Model" >
      <property name="ItemDefinitionId" type="Persistence.NullGuidUserType, Persistence" column="ItemDefinitionId"/>
   </subclass>
  </class>
</hibernate-mapping>
 

We have decided to use Table per class hierarchy, hence the use of a discriminator, but you can obviously choose table per sub-class if that suits you better.

We have also marked the base class as public abstract, as it is abstract ;), but if your base class can be instantiated then you obviously don't want to use that.

I recommend you get a copy of Hibernate In Action for a better expalantion than what I've given.

Just be aware that this book requires *very* careful attention when reading it.

mgaerber · **Posted:** Tue Jan 31, 2006 5:19 am

Alright, thanks for the hint!

Regarding relations, can I easily maintain common ones over the base class?

Let's say, the base table has a many-to-many relation to a categories table,
are those relations available for subclasses as well and how would I set this up?

martijnb · **Joined:** Sat May 14, 2005 8:40 am **Posts:** 130

I have a slight feeling that we're currently thinking about the same concrete problem ;).

Personally I have pretty good experiences with table-per-class mappings (joined subclasses). A table per class hierarchy is impossible, because we're talking about a modular application where every module can introduce its own subclasses with specific attributes.
Another option is to use one table per subclass but that would introduce many duplicate columns in those table, not to mention the fact that you have to map relations to the same classes over and over again.

The only drawback of joined subclasses might be the performance because to getting an instance of a subclass always requires a join. Haven't experienced this as a problem yet.

mgaerber · **Posted:** Wed Feb 01, 2006 1:53 pm

I see that joined subclasses is the way to go.

Thanks for the help (still reading NHibernate docs ;-))