Question about NHibernate

Question

Hi, could somebody please review my source and let me know where I am going wrong. I am trying to insert a new application record and am receiving an exception when I try session.Flush().

Here is the application mapping (have changed some values for display purposes):

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"  assembly="..." namespace="...">
<class name="Application" table="APPLICATION">
<id name="UniqueId" column="COUNTER">
  <generator class="sequence">
        <param name="sequence">MySQ</param>
  </generator>
</id>   
<property name="CreatedDate" column="CREATE_DATE"/>
<property name="CreatedBy" column="CREATE_BY" type="AnsiString"/>
...
<many-to-one name="ApplicationType" column="APPLICATION_CODE" class="ApplicationTypeCode" fetch="join" lazy="false" property-ref="CodeValue"/>
<many-to-one name="ResultCode" column="RESULT_CODE" class="ApplicationResultCode" fetch="join" lazy="false" property-ref="CodeValue"/>
<property name="ApplicationCost" column="COST" type="AnsiString"/>
</class>
</hibernate-mapping>

Here is the class for Application:

using System;
using System.Collections.Generic;
using System.Text;

namespace ...
{
public class Application: ICreation
{
    private int? mUniqueID;

    public virtual int? UniqueId
    {
        get { return mUniqueID; }
        set { mUniqueID = value; }
    }

    private DateTime? createdDate;
    private string createdBy;

    public virtual string CreatedBy
    {
        get { return createdBy; }
        set { createdBy = value; }
    }

    public virtual DateTime? CreatedDate
    {
        get { return createdDate; }
        set { createdDate = value; }
    }

    private ApplicationTypeCode mApplicationType;

    public virtual ApplicationTypeCode ApplicationType
    {
        get { return mApplicationType; }
        set { mApplicationType = value; }
    }

    private ApplicationResultCode mResultCode;

    public virtual ApplicationResultCode ResultCode
    {
        get { return mResultCode; }
        set { mResultCode = value; }
    }

    private string mApplicationCost;

    /// <summary>
    /// Gets or sets the application cost, populated from the application type code
    /// </summary>
    /// <value>The application cost.</value>
    public virtual string ApplicationCost
    {
        get { return mApplicationCost; }
        set { mApplicationCost = value; }
    }

}
}

Here is the class for ICreation:

namespace ...
{
public interface ICreation
{
    string CreatedBy
    {
        get;
        set;
    }

    Nullable<DateTime> CreatedDate
    {
        get;
        set;
    }
}
}

Here is the mapping for codes:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"  assembly="..." namespace="...">
<class name="CodeListItem" table="CODES" discriminator-value="0">
<cache usage="read-only"/>
<composite-id >
  <key-property name="CodeListId" column="APP_DATA_CODE"/>
  <key-property name="CodeValue" column="APP_CODE"/>
</composite-id>
<discriminator column="APP_DATA_CODE" type="Int32"/>
<property name="CodeValue" column="APP_CODE" type="AnsiString"/>
<property name="CodeDescription" column="CODE_DESC" type="AnsiString"/>   
<property name="CodeListId" column="APP_DATA_CODE" type="AnsiString"/>
</class>
<subclass name="ApplicationTypeCode" discriminator-value="225" extends="CodeListItem">
<property name="CodeValue" column="APP_CODE" type="AnsiString"/>
<property name="ApplicationCost" column="PARENT_APP_CODE" type="AnsiString"/>
<property name="ValidFor" column="APP_TEXT" type="AnsiString"/> 
</subclass>
<subclass name="ApplicationResultCode" discriminator-value="325" extends="CodeListItem">
<property name="CodeValue" column="APP_CODE" type="AnsiString"/>
</subclass>
</hibernate-mapping>

The following is the CodeListItem class

namespace ...
{
public class CodeListItem
{
    private string mCodeValue;

    public virtual string CodeValue
    {
        get { return mCodeValue; }
        set { mCodeValue = value; }
    }

    private string mCodeDescription;

    public virtual string CodeDescription
    {
        get { return mCodeDescription; }
        set { mCodeDescription = value; }
    }

    private string mCodeListID;

    public virtual string CodeListId
    {
        get { return mCodeListID; }
        set { mCodeListID = value; }
    }

    private DateTime? mDeactDate;

    public virtual DateTime? DeactDate
    {
        get { return mDeactDate; }
        set { mDeactDate = value; }
    }

    public override bool Equals(object obj)
    {
        bool equal = false;
        CodeListItem item = obj as CodeListItem;
        if(item != null)
        {
            if(item.mCodeListID == this.CodeListId && item.mCodeValue == this.CodeValue)
            {
                equal = true;
            }
        }
        return equal;
    }

}
}

Now the error I get when I try a session.Flush is the following:

Unable to cast object of type 'System.String' to type '........BusinessEntities.CodeListItem'

(replaced the namespaces etc with dots)