Need discriminator recommendation for preexisting schema

cromwellryan · **Joined:** Wed Oct 31, 2007 1:50 pm **Posts:** 13

We have a 3rd party database (Oracle) that we cannot change as the application sitting on top is still very much in use. We are extending it though and have, thus far, been successful at using NHibernate with it.

I have run into a road block with one part. We have a table that represents an idea of Attributes. These attributes have a relationship to either Household, Person or Card (think of that as a Hierarchy in that People have cards and those people make up a household). The problem lies in that the AttributeValue table joins to either Household, Person or Card Id based on a CPHLevel column with either C (card), P (Person ), or H (Household ), but I wouldn't say that there should be three subclasses or joined classes for each permuation

So a select for the card's attribute values would look something like this (manually created):

Code:

select attdef.name, attval.value 
from attributedetail attval
  inner join attributedefinition attdef on attval.attributeid = attdef.attributeid
  inner join card on attval.customerkey = card.id 
where def.cphlevel = 'C' and card.cardnumber = 'ABC123456'

While a households attribute select would look like this:

Code:

select attdef.name, attval.value 
from attributedetail attval
  inner join attributedefinition attdef on attval.attributeid = attdef.attributeid
  inner join household on attval.customerkey = household.id 
where def.cphlevel = 'H' and household.State = 'OH'

What would you recommend the Model and/or hbm files look like? I'm kinda stumped.

paulh · **Posted:** Fri Nov 16, 2007 5:15 am

Haven't tested this, but you could try the following...

Have three Attribute child classes, CardAttribute, PersonAttribute and HouseholdAttribute and set this up as table per hierarchy with the appropriate discriminator values. Don't have a reference on the Attribute class back to the owner however.

Now from the owning class, have a collection of the appropriate Attributes and then map it as follows for Card

Code:

<bag name="Attributes" lazy="true" where="cphlevel='C'">
  <key column="customerkey" />
  <one-to-many class="CardAttribute" />
</bag>

If your NHibernate app isn't going to insert Attributes, but only read them, then you don't need the Attribute child classes - they are there to hold the correct discriminator value on insert.