Map a Strategy Pattern using Fluent NHibernate - nhibernate

Question

Essentially the title of this question explains the essense of what I am trying to do, but to create a contrived example...
I have a class, call it Employee. Employee has an IPaymentBehaviour...
public class Employee
{
IPaymentBehaviour _paymentBehaviour;
protected internal Employee() { /* required by NH */}
public Employee(IPaymentBehaviour paymentBehaviour)
{
_paymentBehaviour = paymentBehaviour;
}
}
This corresponds to a database table like so:
dbo.Employees
-> EmployeeId (primary key)
-> PaymentBehaviourId (foreign key to lookup table)
-> Field1
-> Field2
-> Field3
-> Field4
Depending on the value of PaymentBehaviourId I need to 'inject' a different implementation of IPaymentBehaviour into the Employee object. Depending on which PaymentBehaviour was in use, Field1, 2, 3 or 4 might be needed to create that behaviour.
Can anyone tell me how this would be mapped using Fluent-NHibernate?

Answer 1

table Employees
EmployeeId (primary key)
PaymentBehaviourId (foreign key to PaymentBehaviour)
table PaymentBehaviour
PaymentBehaviourId (pk)
type (discriminator)
Field1
Field2
Field3
Field4
Classes
public class Employee
{
/* ... */
public PaymentBehaviour PaymentBehaviour { get; set; }
}
public class PaymentBehaviourA : IPaymentBehaviour
{
/* ... */
public int Field1 { get; set; }
}
public class PaymentBehaviourB : IPaymentBehaviour
{
/* ... */
public int Field2 { get; set; }
}
I don't know FluentNHibernate enough to tell you how it looks like, but in XML you would specify it like this:
<class name="Employee" table="Employees">
<many-to-one name="PaymentBehaviour" class="IPaymentBehaviour">
</class>
<class name="IPaymentBehaviour" abstract="true" >
<discriminator column="type"/>
<subclass name="PaymentBehaviourA" discriminator-value="A">
<propert name="Field1"/>
</subclass>
<subclass name="PaymentBehaviourB" discriminator-value="B">
<propert name="Field2"/>
</subclass>
</class>

Answer 2

NHibernate uses the default constructor to instantiate entities. There are some workarounds though.

Answer 3

I think I would work around it like this:
Create a default constructur on the
Employee object , that has a
private access modifier, so that NHibernate can reconstruct those types
Create an Interceptor in where you overload the appropriate methods
(OnLoad I presume), in which you make
sure that you inject the correct
IPaymentBehaviour implementation
into the entity. (Perhaps you can
create an interface
'IPaymentBehaviourInjectable' that
can be implemented by the Employee
class (implement it explicitily), so
that you can inject the correct
behaviour into the entity ...