The reference I've used while learning to use Castl'e ActiveRecord implementation is the Blog/Post demos that can be found on Castle's site.

Let's look at the Type Hierarchy example, that can be found here.

It shows an implementation of a class diagram that look a little bit like this:
Class Diagram

Let's look at the code :

[ActiveRecord("entity"), JoinedBase]
public class Entity : ActiveRecordBase
{
   private int _id;
   [PrimaryKey]
   public int Id
   {
      get { return id; }
      set { id = value; }
   }
}

[ActiveRecord("entitycompany")]
public class CompanyEntity : Entity
{
   private int comp_id;
   [JoinedKey("comp_id")]
   public int CompId
   {
      get { return comp_id; }
      set { comp_id = value; }
   }
}

[ActiveRecord("entityperson")]
public class PersonEntity : Entity
{
   private int person_id;
   [JoinedKey("person_id")]
   public int PersonId
   {
      get { return person_id; }
      set { person_id = value; }
   }
}

But look what happens. since the Id property on entity is public and inherited to the subclasses, you get something like this:
Should I use .Id or .PersonId?

So there is a duplicate field here !!!. and it's not only a getter-setter thingie. It also have different private members.

My solution for this is to virtualize the base Id, and protectedize (hehe) the _id member, like this:

[ActiveRecord("entity"), JoinedBase]
public class Entity : ActiveRecordBase
{
   protected int _id;
   [PrimaryKey]
   public virtual int Id
   {
      get { return id; }
      set { id = value; }
   }
}
[ActiveRecord("entitycompany")]
public class CompanyEntity : Entity
{
   [JoinedKey("comp_id")]
   public override int Id
   {
      get { return _id; }
      set { _id = value; }
   }
}
[ActiveRecord("entityperson")]
public class PersonEntity : Entity
{
   [JoinedKey("person_id")]
   public override int Id
   {
      get { return _id; }
      set { _id = value; }
   }
}

 
Now it makes more sence:
I should use .Id

 

And before you hit me with a big stick - I do know of ActiveRecordBase<T> . :) the above code is for demonstration purposes only, not to be Copy&Pasted to your Brand-New-Best-Erp-Ever-Made-And-Will-Make-You-Rich