多对多关系在数据库也是比较常见的，它通过一个中间表将两个主表关联起来。下面来看看多对多关联在nhibernate的实现，示例是一个用户和组之间的多对多关联。先来看看用户类的映射信息: <class name="User, AssemblyName" table="Users">   <id name="UserId" unsaved-value="0" type="int32" column="user_id">      <generator </id>   <property name="Name" type="String" column="name"/>   <set name="Groups" table="UserGroups" inverse="true">      <key column="user_id"/>      <many-to-many AssemblyName" column="group_id"/>   </set></class>在多对多定义中，定义了中间表为UserGroups，此表只有两个字段：user_id和group_id；用于关联Users和Groups表。用户类的定义： public class User {   public User() {   }   public int UserId   {      get { return userId; }      set { userId = value; }   }   public int Name   {      get { return name; }      set { name = value; }   }   public IDictionary Groups   {      get { return groups; }      set { groups = value; }   }   private int userId;   private string name;   private IDictionary groups = new Hashtable();} //class User这里用一个数据字典IDictionary对角来保存组对象。再来看看组类的映射信息： <class name="Group, AssemblyName" table="Groups">   <id name="GroupId" unsaved-value="0" type="int32" column="group_id">      <generator </id>   <property name="Name" type="String" column="name"/>   <property name="description" type="String" column="description"/>   <set name="Users" table="UserGroups">      <key column="group_id"/>      <many-to-many AssemblyName" column="user_id"/>   </set></class>这里many-to-many的定义和用户类映射信息中的差不多。组类的定义： public class Group {   public Group() {   }   public int GroupId   {      get { return groupId; }      set { groupId = value; }   }   public int Name   {      get { return name; }      set { name = value; }   }   public int Description   {      get { return description; }      set { description = value; }   }   public IDictionary Users   {      get { return users; }      set { users = value; }   }   private int userId;   private string name;   private IDictionary groups = new Hashtable();} //class User注意：多对多没有主次之分，保存时的两边都要save。下面给出部分测试代码。 public TestCreate() {   User user1 = new User();   user1.Name = "test1";   User user2 = new User();   user2.Name = "test2";   Group group1 = new Group();   group1.Name = "group1";    Group group2 = new Group();   group2.Name = "group2";   user1.Groups.Add( group2, group2 );   user2.Groups.Add( group1. group1 );   group1.Users.Add( user2, user2 );   group2.Users.Add( user1, user1 );   ITransactioin trans = null;   try {      trans = session.BeginTransaction();      Session.Save( user1 );      Session.Save( user2 );      Session.Save( group1 );      Session.Save( group2 );      trans.Commit();   }   catch ( Exception e ) {      if ( trans != null ) trans.Rollback();      throw e;   }   finally {      session.Close();   }}以上测试代码中session的相关操作请查看相关文档。