一个NHibernate应用的实际例子(A good open source project to learn how to use NHibernate)

此项目名字叫Cuyahoga，是一个CMS。该项目中就使用了NHibernate0.6作为持久层。并且使用了NHibernate0.6新添加的Lazy loading特性，是学习使用NHibernate的上佳例子。
下面是对他的一些评价：
- Use the HttpContext to store your NHibernate session facade. This is what I call the HttpContext Local Session pattern 8-)  
 使用HttpContext .Item来存储NH的Session，亦即使用了HttpRequest--Session的模式

- Access to the session facade is provided by a property get in your overrided ASP.NET page's base class, such as "base.GetCoreRepository.LoadObjectById(..)". This is an alternative to having the repository itself control instantiation such as "CoreRepository.Instance.LoadObjectById(..)"  
 CoreRepository就是一个数据访问逻辑组件了，对Session进行了管理，并且提供数据持久化的方法。部分代码如下：

    public class CoreRepository
    {
        private ISessionFactory _factory;
        private ISession _activeSession;

        /**//// <summary>
        /// Get the active NHibernate session.
        /// </summary>
        public ISession ActiveSession
        {
            get { return this._activeSession; }
        }
        /**//// <summary>
        /// Create a repository for core objects.
        /// </summary>
        public CoreRepository() : this(false)
        {
        }

        /**//// <summary>
        /// Create a repository for core objects.
        /// </summary>
        /// <param name="openSession">Indicate if the CoreRepository should open a session and keep it in memory.</param>
        public CoreRepository(bool openSession)
        {
            this._factory = SessionFactory.GetInstance().GetNHibernateFactory();
            if (openSession)
            {
                this._activeSession = this._factory.OpenSession();
            }
        }

        /**//// <summary>
        /// Open a NHibernate session.
        /// </summary>
        public void OpenSession()
        {
            if (this._activeSession == null || ! this._activeSession.IsOpen)
            {
                this._activeSession = this._factory.OpenSession();
            }
            else
            {
                throw new InvalidOperationException("The repository already has an open session");
            }
        }

        /**//// <summary>
        /// Flushes the current active NHibernate session.
        /// </summary>
        public void FlushSession()
        {
            if (this._activeSession != null && this._activeSession.IsOpen)
            {
                this._activeSession.Flush();
            }
        }

        /**//// <summary>
        /// Close the active NHibernate session
        /// </summary>
        public void CloseSession()
        {
            if (this._activeSession != null && this._activeSession.IsOpen)
            {
                this._activeSession.Close();
            }
        }

        Generic methods#region Generic methods

        /**//// <summary>
        /// Generic method for retrieving single objects by primary key.
        /// </summary>
        /// <param name="type"></param>
        /// <param name="id"></param>
        /// <returns></returns>
        public object GetObjectById(Type type, int id)
        {
            if (this._activeSession != null)
            {
                return this._activeSession.Load(type, id);
            }
            else
            {
                throw new NullReferenceException("The repository doesn't have an active session");
            }
        }

        /**//// <summary>
        /// Get all objects of a given type.
        /// </summary>
        /// <param name="type"></param>
        /// <returns></returns>
        public IList GetAll(Type type)
        {
            return GetAll(type, null);
        }

        /**//// <summary>
        /// Get all objects of a given type and add one or more names of properties to sort on.
        /// </summary>
        /// <param name="type"></param>
        /// <param name="sortProperties"></param>
        /// <remarks>Sorting is Ascending order. Construct a specific query/method when the sort order
        /// should be different.</remarks>
        /// <returns></returns>
        public IList GetAll(Type type, params string[] sortProperties)
        {
            ICriteria crit = this._activeSession.CreateCriteria(type);
            if (sortProperties != null)
            {
                foreach (string sortProperty in sortProperties)
                {
                    crit.AddOrder(Order.Asc(sortProperty));
                }
            }
            return crit.List();
        }

        /**//// <summary>
        /// Generic method to insert an object.
        /// </summary>
        /// <param name="obj"></param>
        public void SaveObject(object obj)
        {
            ITransaction trn = this._activeSession.BeginTransaction();
            try
            {
                // Try to find a UpdateTimestamp property and when found, set it to the current date/time.
                PropertyInfo pi = obj.GetType().GetProperty("UpdateTimestamp");
                if (pi != null)
                {
                    pi.SetValue(obj, DateTime.Now, null);
                }
                this._activeSession.Save(obj);
                trn.Commit();
            }
            catch (Exception ex)
            {
                trn.Rollback();
                throw ex;
            }
        }

        /**//// <summary>
        /// Generic method to update an object.
        /// </summary>
        /// <param name="obj"></param>
        public void UpdateObject(object obj)
        {
            ITransaction trn = this._activeSession.BeginTransaction();
            try
            {
                this._activeSession.Update(obj);
                trn.Commit();
            }
            catch (Exception ex)
            {
                trn.Rollback();
                throw ex;
            }
        }

        /**//// <summary>
        /// Delete a specific object. Settings in the mapping file determine if this cascades
        /// to related objects.
        /// </summary>
        /// <param name="obj"></param>
        public void DeleteObject(object obj)
        {
            ITransaction trn = this._activeSession.BeginTransaction();
            try
            {
                this._activeSession.Delete(obj);
                trn.Commit();
            }
            catch (Exception ex)
            {
                trn.Rollback();
                throw ex;
            }
        }

        /**//// <summary>
        /// Attach potentially stale objects to the current NHibernate session. This is required
        /// when objects are cached in the ASP.NET cache and they contain lazy loaded members.
        /// </summary>
        /// <param name="obj"></param>
        public void AttachObjectToCurrentSession(object obj)
        {
            if (this._activeSession != null)
            {
                if (this._activeSession.IsOpen)
                {
                    this._activeSession.Update(obj);
                }
                else
                {
                    throw new InvalidOperationException("The current NHibernate session is not open, so no objects can be attached.");
                }
            }
            else
            {
                throw new NullReferenceException("No active NHibernate session available to attach the object to.");
            }
        }

        /**//// <summary>
        /// Mark an object for deletion. Commit the deletion with Session.Flush.
        /// </summary>
        /// <param name="obj"></param>
        public void MarkForDeletion(object obj)
        {
            this._activeSession.Delete(obj);
        }

        #endregion
           }

- A new NHibernate Open Session at beginning of a web request and close it at the end of the request. This is the "session-per-request" pattern. 
- Using a custom IHttpModule called NHSessionModule to provide the handlers for Context_BeginRequest and Context_EndRequest. This is a nice way of automatically wiring up your creation/opening and closing and sessions. 
 使用了一个HttpModule 来进行Session的存储和获得，代码如下：

using System;
using System.Web;
using Cuyahoga.Core.Service;

namespace Cuyahoga.Web.Util
{
    在一个HTTP Request期间管理NHibetnate的Session#region 在一个HTTP Request期间管理NHibetnate的Session
    /**//// <summary>
    /// Http module that manages the NHibernate sessions during an HTTP Request.
    /// </summary>
    public class NHSessionModule : IHttpModule
    {
        /**//// <summary>
        /// Default constructor.
        /// </summary>
        public NHSessionModule()
        {
        }

        public void Init(HttpApplication context)
        {
            context.BeginRequest += new EventHandler(Context_BeginRequest);
            context.EndRequest += new EventHandler(Context_EndRequest);
        }

        public void Dispose()
        {
            // Nothing here    
        }

        private void Context_BeginRequest(object sender, EventArgs e)
        {
            // Create the repository for Core objects and add it to the current HttpContext.
            CoreRepository cr = new CoreRepository(true);
            HttpContext.Current.Items.Add("CoreRepository", cr);
        }

        private void Context_EndRequest(object sender, EventArgs e)
        {
            // Close the NHibernate session.
            if (HttpContext.Current.Items["CoreRepository"] != null)
            {
                CoreRepository cr = (CoreRepository)HttpContext.Current.Items["CoreRepository"];
                cr.CloseSession();
            }
        }
    }
    #endregion
}

也就是将session保存到HttpContext.Current.Items中，这样做的好处请看这里。

NHibernate中添加的最重要的特性就是用proxy对延迟加载的支持（包括对集合的和单个object的延迟加载），这也是因为项目中使用了动态代理技术（这要求业务实体的成员需要是virtual的或者实现某个接口）， 这为多表的join带来了性能上的提升。