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copied!<blockquote> <p>I think this should be Many-to-Many relationship</p> </blockquote> <p>You are correct. In your scenario you will need to use a self-referential many-to-many mapping. But using a single <em>Users</em> table you cannot represent the relation between <em>users</em> and <em>friends</em> (using a single table you could represent a self-referential <a href="http://www.hibernate.org/hib_docs/nhibernate/1.2/reference/en/html_single/#example-parentchild" rel="noreferrer">parent-child</a> relationship). You will need an intermediary table to achieve this. Here's an example using <a href="http://www.sqlite.org/" rel="noreferrer">SQLite</a> ADO.NET <a href="http://sqlite.phxsoftware.com/" rel="noreferrer">provider</a> to show one possible way of modeling your scenario:</p> <p>User.hbm.xml:</p> <pre><code><?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Test" assembly="test"> <class name="User" table="Users"> <id name="Id" column="id"> <generator class="native"/> </id> <property name="Name" column="name"/> <set name="Friends" table="Friends"> <key column="user_id"/> <many-to-many class="User" column="friend_id"/> </set> </class> </hibernate-mapping> </code></pre> <p>From the above mapping you will notice the use of the following tables: Users and Friends</p> <p>And here's the code:</p> <pre><code>using System; using System.IO; using System.Collections.Generic; using System.Data.SQLite; using NHibernate; using NHibernate.Cfg; using Iesi.Collections.Generic; namespace Test { class Program { public static void Main() { if (File.Exists("nhibernate.db")) { File.Delete("nhibernate.db"); } ExecuteCommand("create table Users (id integer, name string)"); ExecuteCommand("create table Friends (user_id integer, friend_id string)"); ExecuteCommand("insert into Users (id, name) values (1, 'user1')"); ExecuteCommand("insert into Users (id, name) values (2, 'user2')"); ExecuteCommand("insert into Users (id, name) values (3, 'user3')"); // User1 is friend with User2 ExecuteCommand("insert into Friends (user_id, friend_id) values (1, 2)"); // User1 is friend with User3 ExecuteCommand("insert into Friends (user_id, friend_id) values (1, 3)"); // User2 is friend with User1 ExecuteCommand("insert into Friends (user_id, friend_id) values (2, 1)"); // User3 is friend with User1 ExecuteCommand("insert into Friends (user_id, friend_id) values (3, 1)"); ISessionFactory sessionFactory = new Configuration().Configure().BuildSessionFactory(); ISession session = sessionFactory.OpenSession(); User user = session.Get<User>(1); Console.WriteLine(user.Friends.Count); session.Close(); sessionFactory.Close(); } private static void ExecuteCommand(string sql) { using (SQLiteConnection connection = new SQLiteConnection("Data Source=nhibernate.db;Version=3")) using (SQLiteCommand command = new SQLiteCommand(sql, connection)) { connection.Open(); command.ExecuteNonQuery(); } } } class User { public virtual int Id { get; set; } public virtual string Name { get; set; } public virtual ISet<User> Friends { get; set; } } } </code></pre> <p>And finally for the sake of completeness here's my config file:</p> <pre><code><?xml version="1.0" encoding="utf-8" ?> <configuration> <configSections> <section name="hibernate-configuration" type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate" /> </configSections> <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2"> <session-factory> <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property> <property name="connection.driver_class">NHibernate.Driver.SQLite20Driver</property> <property name="dialect">NHibernate.Dialect.SQLiteDialect</property> <property name="connection.connection_string">Data Source=nhibernate.db;Version=3</property> <mapping assembly="test" /> </session-factory> </hibernate-configuration> </configuration> </code></pre>

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