Mapping legacy database with possibly a discriminator - nhibernate

Question

I am trying to map a legacy database. I need to implement what I believe should be a discriminator. My problem is discriminators only seem to work when there is a column to differentiate or a formula on the current row. For my case there is no actual differentiator, the data is either joined to one table if it exists, if not, then the other table. To make things even more complicated the table uses a composite key.
Here's an example (it might be oversimplified as I am making it up):
Given my code
public class SomeTable {
public virtual int DataID { get; set; }
public virtual int EmployeeOrCustomer { get; set; }
public virtual Person Person { get; set; }
public virtual int SomeValue { get; set; }
}
public abstract class Person {
public virtual int ID { get; set; }
public virtual string Name { get; set; }
}
public class Employee : Person {
public virtual int EmployeeNumber { get; set; }
}
public class Customer : Person {
public virtual int CustomerNumber { get; set; }
}
And my Data
-SomeTable-
DataID(K) EmployeeOrCustomer(K) SomeValue
1 1 100
1 22 222
-Employee-
ID Name EmployeNumber
1 Joe Blow 12345
-Customer-
ID Name CustomerNumber
22 ACME Inc. 4242
My Mappings:
<class name="SomeTable" abstract="true">
<composite-id>
<key-property name="DataID" />
<key-property name="EmployeeOrCustomer" />
</composite-id>
<property name="SomeValue" />
<!-- ?????? -->
</class>
<class name="Employee">
<id name="ID" />
<Property name="EmployeeNumber">
</Class>
<class name="Customer">
<id name="ID" />
<Property name="CustomerNumber">
</Class>
What I expect
DataID: 1, SomeValue: 100 Person: { Employee: EmployeeNumber: 12345 }
DataID: 1, SomeValue: 222, Person: { Customer: CustomerNumber: 4242 }
I thought about joining the 2 tables and using the merged tables as a join but the 2 tables differ a lot. Also, I can't modify the schema so adding a discriminator column is not an option.
Any ideas?

Answer 1

i think what you are searching for is union subclass inheritance mapping
<class name="Person" abstract="true">
<id name="Id">
<generator class="assigned"/>
</id>
<union-subclass table="Employee" name="Employee">
<property name="Name"/>
<property name="EmployeeNumber"/>
</union-subclass>
<union-subclass table="Customer" name="Customer">
<property name="Name"/>
<property name="CustomerNumber"/>
</union-subclass>
</class>
<class name="SomeTable" abstract="true">
<composite-id>
<key-property name="DataID" />
<key-many-to-one name="Person" column="EmployeeOrCustomer" />
</composite-id>
<property name="SomeValue" />
</class>
Note the extra property EmployeeOrCustomer is not needed.