public class User {
    virtual public long Id {get;set;}
    virtual public string Name {get;set;}
}

public class Computer {
    virtual public long Id {get;set;}
    virtual public string Name {get;set;}
}

public class UserToComputer {
    virtual public User {get;set;}
    virtual public Computer {get;set;}
}

<class name="User">
  <id name="Id">
    <generator class="identity"/>
  </id>
  <property name="Name"/>
</class>

<class name="Computer">
  <id name="Id">
    <generator class="identity"/>
  </id>
  <property name="Name"/>
</class>

<class name="UserToComputer">
  <composite-id>
    <key-many-to-one name="User" column="UserId" class="User"/>
    <key-many-to-one name="Computer" column="ComputerId" class="Computer"/>
  </composite-id>
</class>

var user = new User {Name = "User 1"};
var computer = new Computer {Name = "Computer 1"};
var userToComputer = new UserToComputer {User = user; Computer = computer;}

using (var session = sessionFactory.OpenSession())
{
    session.Save(userToComputer);
    session.Flush();
}

var user = new User {Name = "User 1"};
var computer = new Computer {Name = "Computer 1"};
var userToComputer = new UserToComputer {User = user; Computer = computer;}

using (var session = sessionFactory.OpenSession())
{
    session.SaveOrUpdate(user);
    session.SaveOrUpdate(computer);
    session.Save(userToComputer);
    session.Flush();
}