<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Test_NHibernate_Join_Tables" assembly="Test_NHibernate_Join_Tables"
                  >
  <class name="Test_NHibernate_Join_Tables.Products" table="tblProducts">
    <id name="ProductId" type="int">
      <generator class="native"/>
    </id>
    <property type="string" name="ProductName" >
      <column name="ProductName"></column>
    </property>
    <set name="Category" inverse="true" cascade="all" >
      <key column="CategoryId" />
      <one-to-many class="Test_NHibernate_Join_Tables.Products"/>
    </set>
  </class>

</hibernate-mapping>

public class Products
    {

        public virtual int? ProductId { get; set; }
        Category _category;
        public virtual Category Category
        {
            get
            {
                return _category;
            }
            set
            {
                _category = value;
            }
        }
        public virtual string ProductName { get; set; }
    }