I have this object :

public class Ledger
{
  public virtual Person Client { get; set; }
  // ....
}

The Ledger table joins to the Person table via one of three possible tables : Bill, Receipt or Payment.

So we have the following tables :

Ledger LedgerID PK

Bill BillID PK, LedgerID, ClientID

Receipt ReceiptID PK, LedgerID, ClientID

Payment PaymentID PK, LedgerID, ClientID

If it was just the one table, I could map this as :

Join ( "Bill", x =>
{
    x.ManyToOne ( ledger => ledger.Client, mapping => mapping.Column ( "ClientID" ) );
    x.Key ( l => l.Column ( "LedgerID" ) );
} );

This won't work for three tables. For a start the Join performs an inner join. Since there will only ever be one of Bill, Receipt or Payment - an inner join on these tables will always return zero rows.

Then it would need to know to do a Coalesce on the ClientID of each of these tables to know the ClientID to grab the data from.

Is there a way to do this? Or am I asking too much of the mappings here?