使用NHibernate对延迟加载的集合进行分页

Question

我读过这篇文章,其中Ayende表示NHibernate可以(与EF 4相比):

• 使用lazy ="extra"的集合 - Lazy extra意味着NHibernate适应您可能在集合之上运行的操作.这意味着blog.Posts.Count不会强制加载整个集合,而是会创建一个"从Posts中选择count(*)BlogId = 1"语句,并且blog.Posts.Contains()同样会产生在单个查询中,而不是支付将整个集合加载到内存的价格.
• 集合过滤器和分页集合 - 这允许您在实体集合之上定义其他过滤器(包括分页!),这意味着您可以轻松浏览blog.Posts集合,而不必将整个内容加载到内存中.

所以我决定整理一个测试案例.我创建了陈词滥调的Blog模型作为一个简单的演示,有两个类如下:

public class Blog
{
    public virtual int Id { get; private set;  }
    public virtual string Name { get; set; }

    public virtual ICollection<Post> Posts { get; private set;  }

    public virtual void AddPost(Post item)
    {
        if (Posts == null) Posts = new List<Post>();
        if (!Posts.Contains(item)) Posts.Add(item);
    }
}

public class Post
{
    public virtual int Id { get; private set; }
    public virtual string Title { get; set; }
    public virtual string Body { get; set; }
    public virtual Blog Blog { get; private set; }
}

我的映射文件如下所示:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="property" auto-import="true" default-cascade="none" default-lazy="true">
  <class xmlns="urn:nhibernate-mapping-2.2" name="Model.Blog, TestEntityFramework, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="Blogs">
    <id name="Id" type="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Id" />
      <generator class="identity" />
    </id>
    <property name="Name" type="System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Name" />
    </property>
    <property name="Type" type="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Type" />
    </property>
    <bag lazy="extra" name="Posts">
      <key>
        <column name="Blog_Id" />
      </key>
      <one-to-many class="Model.Post, TestEntityFramework, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
    </bag>
  </class>
</hibernate-mapping>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="property" auto-import="true" default-cascade="none" default-lazy="true">
  <class xmlns="urn:nhibernate-mapping-2.2" name="Model.Post, TestEntityFramework, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="Posts">
    <id name="Id" type="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Id" />
      <generator class="identity" />
    </id>
    <property name="Title" type="System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Title" />
    </property>
    <property name="Body" type="System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Body" />
    </property>
    <many-to-one class="Model.Blog, TestEntityFramework, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" name="Blog">
      <column name="Blog_id" />
    </many-to-one>
  </class>
</hibernate-mapping>

我的测试用例看起来像这样:

        using (ISession session = Configuration.Current.CreateSession()) // this class returns a custom ISession that represents either EF4 or NHibernate
        {
            blogs = (from b in session.Linq<Blog>()
                         where b.Name.Contains("Test")
                         orderby b.Id
                         select b);

            Console.WriteLine("# of Blogs containing 'Test': {0}", blogs.Count());
            Console.WriteLine("Viewing the first 5 matching Blogs.");

            foreach (Blog b in blogs.Skip(0).Take(5))
            {
                Console.WriteLine("Blog #{0} \"{1}\" has {2} Posts.", b.Id, b.Name, b.Posts.Count);
                Console.WriteLine("Viewing first 5 matching Posts.");

                foreach (Post p in b.Posts.Skip(0).Take(5))
                {
                    Console.WriteLine("Post #{0} \"{1}\" \"{2}\"", p.Id, p.Title, p.Body);
                }
            }
        }

使用lazy ="extra",调用b.Posts.Count确实做了一个SELECT COUNT(Id)...很棒的事情.但是,b.Posts.Skip(0).Take(5)只需获取Blog.Id =？id的所有帖子,然后应用程序端的LINQ只是从结果集合中获取前5个.

是什么赋予了？

Answer 1

我非常肯定(阅读评论)他正在谈论ISession的CreateFilter.

您可以像这样进行分页(来自文档13.13):

Collections are pageable by using the IQuery interface with a filter:

IQuery q = s.CreateFilter( collection, "" ); // the trivial filter
q.setMaxResults(PageSize); 
q.setFirstResult(PageSize * pageNumber); 
IList page = q.List();

或者(来自文档17.1.4):

s.CreateFilter( lazyCollection, "").SetFirstResult(0).SetMaxResults(10).List();

这不像使用System.Linq方法那么顺利.我猜他们也会加入语法一段时间.