Fluent nHibernate, Auto-Incrementing Numeric other than a Key

I have a situation where say, a Notebook stores Pages. At first, I had it structured like this.

class Notebook
{
 // ...
 public virtual IList<Page> Pages
 {
  get;
  set;
 }
}

class Page
{
 // ..
 public virtual int Number
 {
  get;
  set;
 }
}

That worked okay. Each page had a page number. I would have the ability to re-order pages in my UI, and update the number. No problem. Then I discovered I could use a Dictionary, and simplify it more...

class Notebook
{
 // ...
 public virtual IDictionary<int, Page> Pages
 {
  get;
  set;
 }
}

class Page
{
 // ..
}

And map it like so..

public class NotebookMap : ClassMap<Notebook>
{
    public NotebookMap()
    {
        // Specify the Entity Primary Key
        Id(x => x.Id);

        // one notebook wil开发者_高级运维l have a list of pages with page numbers
        HasMany<int, Page>(x => x.Pages)
            .DictionaryKey<int>("Number")
            .DictionaryValue<Page>("Page")
            .ForeignKey("Notebook")
            .Schema("Notebook").Table("Pages"); 

        Schema(Schemas.Collections ); Table("Notebooks");
    }
}

Huzzah, everything was right with the world. This works great. But I'm curious...

Is there any way to bind Number to something that will make sure to auto-increment per Notebook? Right now, I just handle this in my Service by using the Notebook.Pages.Count method when I add a new page. But fluent nHibernate seems pretty smart. Is there any way I can integrate it right into the mapping, so that anytime I add a new page, it just gets the count and makes sure the number is appropriate?

I think you really want to keep this out of the mapping. You'd be mixing the application logic (Where should the next page go) with the model (how am I storing the next page). Also if you make this column auto-increment you'd have to do it on a per table basis. Unless you are creating a new table for each book, you'd (AFAIK) only be able to have 1 consistently incrementing key (as the native generator which uses auto increment columns in NHibernate is done via the database).