how to map inherited set of classes to db in NHibernate?

Question

I have an inherited class structure with multiple table. My class structure is as follows. (This is not the real classes. I have created fake classes which imitate the originals here to save space.)

class Account {
    public virtual int AID { get; set; }
    public virtual string Name { get; set; }
    public virtual string Serial { get; set; }
    public virtual string Type { get; set; }

    public Account { Type = "Default"; }
}

class Human : Account {
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
    // more properties..

    public Human { Type = "Human"; }
}

class Bank : Account {
    public virtual string BranchName { get; set; }
    // more properties..

    public Bank { Type = "Bank"; }
}

My Account.hbm.xml file looks like below.

  <class name="Account">
    <id name="AID" type="Int32">
      <generator class="native" />
    </id>
    <discriminator column="Type" type="string" />
    <property name="Name" />
    <property name="Serial" />
    <property name="Type" />

    <subclass name="Human" discriminator-value="Human">
      <join table="Account">
        <key column="AID" />
        <property name="FirstName" />
        <property name="LastName" />
      </join>
    </subclass>

    <subclass name="Bank" discriminator-value="Bank">
      <join table="Account">
        <key column="AID" />
        <property name="LocationName" />
      </join>
    </subclass>    
  </class>

I execute this and tables are created with the correct set of fields. Problem is when I insert the record.

I do it as of below.

Bank bank = new Bank();
bank.Name = "Any Name";
bank.Serial = "0001";
bank.BranchName = "Local CIty";

using (ISession session = NHibernateHelper.OpenSession())
{
    using (ITransaction transaction = session.BeginTransaction())
    {
        session.Save(bank);
        transaction.Commit();
    }
}

Below is the error produced by NUnit. Maybe I am doing somehting wrong. Can any one help me on this please?

NHibernate.PropertyValueException : Error dehydrating property value for 
xxxx.xxxxx.Bank.Name
  ----> System.IndexOutOfRangeException : Parameter index is out of range.

Answer 1

I modified the Account.hbm.xml mapping file with cluase. Now the things work fine. I can smoothly insert records now without any errors.

Here is the working mapping file.

  <class name="Account">
    <id name="AID" type="Int32">
      <generator class="native" />
    </id>
    <!-- <discriminator column="Type" type="string" /> -->
    <property name="Name" />
    <property name="Serial" />
    <property name="Type" />

    <joined-subclass name="Human" table="Human">
      <key column="AID" />
      <property name="FirstName" />
      <property name="LastName" />
    </subclass>

    <joined-subclass name="Bank" table="Bank">
      <key column="AID" />
      <property name="LocationName" />
    </subclass>    
  </class>

I thought may be discriminator clause might be helpful and overused. :) Without it works fine.