Common with Fluent and NHibernate

I am trying to create a messaging system between users and organizations, i.e. a user can send and receive messages from users or organizations.

So, to support this system, I started with the interface:

public interface IPassableObject<F, T, O>
    where F : DomainObject 
    where T : DomainObject
{
    Guid PassedItemId { get; set; }
    O Item { get; set; }
    F From { get; set; }
    T To { get; set; }
    DateTime Created { get; set; }
}

      

        
        
        
      

    

The idea is that later on I would like to transmit more than just messages - for example, serialized objects. So we have a class with 3 Generic types. To, From and Object

Then I created a Message class

public class Message<T, F> : DomainObject, IPassableObject<T, F, string>
    where F : DomainObject
    where T : DomainObject
{

    public Message()
    {
        Created = DateTime.Now;
    }

    #region IPassableObject<T,F> Members

    public virtual Guid PassedItemId { get; set; }

    public virtual string Item { get; set; }

    public virtual T From { get; set; }

    public virtual F To { get; set; }

    public virtual DateTime Created { get; set; }

    #endregion
}

      

        
        
        
      

    

Then I made a table to support my passed items:

CREATE TABLE dbo.PassedItems
(
    PassedItemId            uniqueidentifier        NOT NULL,
    FromId                  uniqueidentifier        NOT NULL,
    ToId                    uniqueidentifier        NOT NULL,
    SerializedObject        varchar(max)            NOT NULL,
    Created                 datetime                DEFAULT(GETEDATE()) NOT NULL,

    CONSTRAINT PK_PassedItems PRIMARY KEY CLUSTERED (PassedItemId)
)

      

        
        
        
      

    

The id for my users and organizations is sheets. I now have a clear mapping for User-User posts. My thought is that I would only need to create 4 mappings and not maintain 4 separate classes for users, users, organizations, org-users and org-org. I was hoping Nhibernate would take the manual in the table and automatically try to pull the object based on the type specified.

public class UserToUserMessageMap : ClassMap<Message<UserProfile, UserProfile>>
{
    public UserToUserMessageMap()
    {
        WithTable("PassedItems");

        Id(x => x.PassedItemId, "PassedItemId").GeneratedBy.Guid();

        Map(x => x.Item, "SerializedObject");
        Map(x => x.Created);

        References(x => x.From, "FromId");
        References(x => x.To, "ToId");
    }
}

      

        
        
        
      

    

The problem I see is that when I start the application, I get a runtime error:

NHibernate.MappingException: Association from PassedItems table does not specify a referenced object

So, will this approach work? What am I doing wrong? Thank!