NHibernate one to many with discriminator

Question

I'm having an issue mapping what I would think is a fairly simple association.

Here's an example of the domain model I'm trying to map:

public class MyClass
{
    IDictionary<string, DateTime> Dates { get; set; }
}


public class MyOtherClass
{
    IDictionary<string, DateTime> Dates { get; set; }
}

I'd like the Dates property to be mapped into a single table, something like this:

TABLE Dates
COLUMNS
    ParentId    (Parent class key value)
    ParentType  (Parent class type)
    DateType    (Index value for dictionary)
    DateValue
SAMPLE
ParentId        ParentType  DateType    DateValue
----------------------------------------------------------
1               MyClass             TYPEA       2009-10-09
1               MyOtherClass        TYPEA       2009-11-08

I'm open to suggestions as to how to better model this, but basically I want a one to many association without littering the database with a separate table for every collection.

I'm using DateTime in this particular example as that's what I'm trying to do right now, but it could just as easily be any type, even another custom class that I would want to model in the same way.

Any idea as to how this should be mapped?

I believe this is similar to a question I had asked previously (http://stackoverflow.com/questions/722550/nhibernate-how-do-i-map-mutiple-parents-children-of-a-common-type-into-a-single) but that solution ended up in crosstables all over the place which isn't really the result I'm trying to achieve.

Answer 1

Discrimination must be between MyClass and MyOtherClass, not in the Dates table. You need a database structure like this:

Table Classes (for MyClass/MyOtherClass):
    ClassID (unique primary key)
    ClassType
    ...

Table Dates:
    ClassID (foreign key from above)
    Date (value)

Now, you need an abstract base class MyBaseClass from which MyClass and MyOtherClass derive, and your mapping would then be something like this (pseudo-code, don't expect it to be entirely correct):

<class="MyBaseClass", abstract="true", table="Classes">
  <id .../>
  <set name="Dates"... />
  <discriminator column="ClassType"/>
  <subclass type="MyNamespace.MyClass, MyAssembly" discriminator-value="'MyClass'">
    ...
  </subclass>
  <subclass type="MyNamespace.MyOtherClass, MyAssembly" discriminator-value="'MyOtherClass'">
    ...
  </subclass>
</class>

HTH!