Any advice designing self-referencing class?

fly2moon2 · **Joined:** Mon May 22, 2006 12:12 am **Posts:** 23

Hi all,

Any idea on designing self-referencing class? i.e. class pointing to itself.

e.g. A (big) District, is made up of a number of (smaller) Districts.

There will be a single table named, District (right to do it this way?)

so, it is one-to-many parent-child relationship.

How should I design in NHibernate the hbm.xml file?

best wishes,
jL

gert · **Joined:** Thu Jan 19, 2006 4:29 pm **Posts:** 348

Why do You think that the self reference should be different from reference to another class? Have You got any issues? As far as I remember I didn't got any...

Gert

devonl · **Posted:** Fri May 26, 2006 12:02 pm

a single table is correct. but the ParentDistrictID has to be nullable:

Code:

T_District
DistrictID       [ Identity ]
DistrictName     [ varchar(50) ]
ParentDistrictID [ Nullable ]

Your class might look like:

Code:

public class District() {
   private int id;
   private string name;
   private District parentDistrict;
   privte IList childrenDistricts;

   ... properties ...
}

Class mapping:

Code:

<class name=District lazy=true>
   <id name="id" ... >
       generator class="identity" />
   </id>

   <property name="Name" column="DistrictName" type=string not-null=true />

   <many-to-one name="ParentDistrict" class="district" column="ParentDistrictID" />

</class>

i definitely recommend marking the association as lazy otherwise you might load an entire object graph that you don't want.

then in your DAO you can get all children like so:

Code:

return session.CreateCriteria(typeofDistrict))
   .Add(Expression.Eq("ParentDistrict", parentDistrict))
   .AddOrder(Order.Asc("Name"))
   .List();

wheee! :)

devonl · **Posted:** Fri May 26, 2006 2:04 pm

actually, i was thinking about this more and i'm wondering if anyone has successfully implemented the Composite pattern in NH? i't might apply here...

antoska · **Posted:** Mon May 29, 2006 4:39 am

I implemented one month ago a "SelfJoin" class in my solution as devonl did.
It works without problems :)

Antonella

devonl · **Posted:** Wed Jun 07, 2006 12:24 pm

found this in the Hibernate docs. easily ported to NH/.NET:

http://www.hibernate.org/86.html

jemiller · **Posted:** Wed Jun 07, 2006 4:05 pm

I was able to implement an Item object which had Items as children. So far, it's just a unidirectional relationship, but, it seems to work OK. i.e. I didn't have to do anything special. It just works the same way it does if I was mapping to another class.