Question

Please help! I couldn’t figure it out how to map the following situation:

I have only 1 table.

[Table] User { id, name }

My class look like this

public class User {   public int Id { get; set; }   public string Name { get; set; }    public ISet<User> Friends { get; set; } } 

Each user has relationship with other users. e.g.’User A’ can has many friends which is other User.

What should be the mapping for this? I think this should be Many-to-Many relationship but I don’t really know how the HBM will look like?

Thanks,

Answer 1

I think this should be Many-to-Many relationship

You are correct. In your scenario you will need to use a self-referential many-to-many mapping. But using a single Users table you cannot represent the relation between users and friends (using a single table you could represent a self-referential parent-child relationship). You will need an intermediary table to achieve this. Here’s an example using SQLite ADO.NET provider to show one possible way of modeling your scenario:

User.hbm.xml:

<?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> 

From the above mapping you will notice the use of the following tables: Users and Friends

And here’s the 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; }     } } 

And finally for the sake of completeness here’s my config file:

<?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>