An example of how to get NHibernate to spit out database schema update scripts.

create_update_script.cs

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.IO;
using System.Linq;
using System.Text;
using NHibernate.Dialect;
using NHibernate.Tool.hbm2ddl;
using StructureMap.Attributes;

namespace SchemaUpdateGenerator
{
    class Program
    {
        static void Main(string[] args)
        {
            NHibernate.Cfg.Configuration cfg = null;

			//Configure NHibernate here

            var dialect = Dialect.GetDialect(cfg.Properties);
            string[] schemaUpdateScript;
            using (var conn = new SqlConnection(
                    cfg.GetProperty("connection.connection_string")))
            {
                conn.Open();
                schemaUpdateScript = cfg.GenerateSchemaUpdateScript(dialect,
                                          new DatabaseMetadata(conn, dialect));
            }

            var f = new FileInfo("schema_update.sql");
            using (var stream = f.OpenWrite())
            {
                var writer = new StreamWriter(stream);
                foreach (var s in schemaUpdateScript)
                {
                    writer.WriteLine(s);
                    writer.WriteLine("GO;");
                    Console.WriteLine(s);
                    Console.WriteLine("GO;");
                }
                writer.Flush();
                stream.Flush();
            }
        }
    }
}