public class User
{
    Guid _id;
    public Guid Id { get { return this._id; } }

    IList _roles = new ArrayList();
    public IList Roles { get { return _roles; } }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" schema="dbo">
    <class name="Security.User,Security" table="Users">
        <id name="Id" type="Guid" >
            <generator class="guid" />
        </id>
        <set name="Roles" table="Users_Roles" lazy="true">
            <key column="UserId" />
            <composite-element class="string" >
                <property name="value" /> <!-- ??? -->
            </composite-element>
        </set>
    </class>