public class Category : DomainObject<long>
{
    public Category()
    {
        ChildCategories = new List<Category>();
    }

    public virtual string Name { get; set; }

    public virtual Category Parent { get; set; }
    public virtual IList<Category> ChildCategories { get; private set; }

    public virtual void AddChild(Category category)
    {
        if (category.Parent !=null && category.Parent != this)
        {
            category.Parent.RemoveChild(category);
        }

        ChildCategories.Add(category);
        category.Parent = this;
    }

    public virtual void RemoveChild(Category category)
    {
        if (ChildCategories.Contains(category))
        {
            ChildCategories.Remove(category);
            category.Parent = null;
        }
    }
}

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateLab.Domain" namespace="NHibernateLab.Domain" default-lazy="true">

  <class name="Category" table="Category">
   
    <id name="Id" column="CategoryID" type="long">
      <generator class="native"/>
    </id>
    
    <property name="Name" column="Name" type="string"/>

    <many-to-one name="Parent" class="Category" column="ParentId"/>
    
    <bag name="ChildCategories" cascade="all" inverse="true">
      <key column="ParentId"/>
      <one-to-many class="Category"/>  
    </bag>
    
  </class>
  
</hibernate-mapping>

public Category GetFullTree(long Id)
{
    string query = @"WITH Hierachy(CategoryId, Name, Level)
                        AS
                        (
	                        SELECT CategoryID, Name, 0 AS Level
	                        FROM Category c
	                        WHERE c.CategoryID = :id
	
	                        UNION ALL
	
	                        SELECT cat.CategoryId, cat.Name, hier.Level + 1
	                        FROM Category cat
	                        INNER JOIN Hierachy hier
	                        ON cat.ParentId = hier.CategoryId
                        )
                        SELECT CategoryId, Name
                        FROM Hierachy
                        WHERE Level >= 0";


    List<Category> list = _session.CreateSQLQuery(query)
                    .AddEntity(typeof (Category))
                    .SetInt64("id", Id)
                    //.SetCacheable(true)
                    .List<Category>().ToList();

    Category result = list.Where(cat => cat.Parent == null).First();

    return result;
}

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateLab.Domain" namespace="NHibernateLab.Domain" default-lazy="true">

  <class name="Category" table="Category">
    <cache usage="read-write"/>  
.......

<hibernate-configuration  xmlns="urn:nhibernate-configuration-2.2" >
  <session-factory name="NHibernate.Test">
.........
    <property name="cache.provider_class">NHibernate.Cache.HashtableCacheProvider</property>
    <property name="cache.use_second_level_cache">true</property>
.........

public void GetCategory(long id)
{
    Category category = _session.CreateQuery(
        "select cat from Category cat join fetch cat.ChildCategories")
        .SetResultTransformer(new DistinctRootEntityResultTransformer())
        .List<Category>()
        .Where(x => x.Id == id)
        .First();
}

select category0_.CategoryID  as CategoryID1_0_,
       category1_.CategoryID  as CategoryID1_1_,
       category0_.Name        as Name1_0_,
       category0_.ParentId    as ParentId1_0_,
       category1_.Name        as Name1_1_,
       category1_.ParentId    as ParentId1_1_
from   Category category0_
       inner join Category category1_
         on category0_.ParentId = category1_.CategoryID

Category tree;

using (ISession session = CreateSession())
{
    int Id = CreateAndPersistCategoryTree();
    tree = LoadCategoryTree(Id);
}

WriteTreeRecursive(tree);

USE [master]
GO

IF EXISTS(SELECT name FROM sys.databases
     WHERE name = 'NHCategories')
     BEGIN
		 DROP DATABASE NHCategories
	 END
GO

CREATE DATABASE NHCategories
GO

USE [NHCategories]
GO
/****** Object:  Table [dbo].[Categories]    Script Date: 02/08/2011 14:25:20 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Categories](
	[Id] [int] IDENTITY( 1 , 1 ) NOT NULL,
	[Name] [nvarchar](max) NOT NULL,
	[ParentId] [int] NULL,
 CONSTRAINT [PK_Categories] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
USE [master]
GO