namespace Care.Inventory {

    public class Person {

        private int id;
        private string name;
        private Author author;

        public Person() {
            author = new Author();
        }
       
        public virtual int Id {
            get { return id; }
            set { id = value; }
        }

        public virtual string Name {
            get { return name; }
            set { name = value; }
        }

        public virtual Author Author {
            get { return author; }
            set { author = value; }
        }
    }
}

namespace Care.Inventory {

    public class Author {

        private int id;
        private string alias;
        private Person person;

        public virtual int Id {
            get { return id; }
            set { id = value; }
        }

        public virtual string Alias {
            get { return alias; }
            set { alias = value; }
        }

        public virtual Person Person {
            get { return person; }
            set { person = value; }
        }

    }
}

CREATE TABLE [dbo].[Person](
   [Id] [int] IDENTITY(1,1) NOT NULL,
   [Name] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED
(
   [Id] ASC
)

CREATE TABLE [dbo].[Author](
   [Id] [int] IDENTITY(1,1) NOT NULL,
   [Alias] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
CONSTRAINT [PK_Author] PRIMARY KEY CLUSTERED
(
   [Id] ASC
)WITH (PAD_INDEX  = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]

GO
ALTER TABLE [dbo].[Author]  WITH CHECK ADD  CONSTRAINT [FK_Author_Person] FOREIGN KEY([Id])
REFERENCES [dbo].[Person] ([Id])
GO

// insert
            Person p = new Person();
            p.Name = "Manoj";

            Author a = new Author();
            a.Alias = "Waikar";
            a.Person = p;
            p.Author = a;

            personDao.Save(p);

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="Care.Inventory.Author, Care.Inventory" table="Author">

      <id name="Id" column="Id" type="Int32">
         <generator class="assigned" />
      </id>

      <property name="Alias" column="Alias" />
      <one-to-one name="Person" class="Care.Inventory.Person, Care.Inventory" constrained="true" />

   </class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="Care.Inventory.Person, Care.Inventory" table="Person">

      <id name="Id" column="Id" type="Int32">
         <generator class="native" />
      </id>

      <property name="Name" column="Name" />
      <one-to-one name="Author" class="Care.Inventory.Author, Care.Inventory" />

   </class>
</hibernate-mapping>

      <one-to-one name="Author" class="Care.Inventory.Author, Care.Inventory" cascade="save-update" />

CREATE TABLE [dbo].[Product](
   [Id] [int] IDENTITY(1,1) NOT NULL,
   [Name] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
   [Description] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
   [CompanyId] [int] NOT NULL,
CONSTRAINT [PK_Product] PRIMARY KEY CLUSTERED
(
   [Id] ASC
)WITH (PAD_INDEX  = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]


CREATE TABLE [dbo].[Stock](
   [Id] [int] NOT NULL,
   [Quantity] [int] NOT NULL,
CONSTRAINT [PK_Stock] PRIMARY KEY CLUSTERED
(
   [Id] ASC
)WITH (PAD_INDEX  = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="Care.Inventory.Product, Care.Inventory" table="Product">

      <id name="Id" type="Int32" unsaved-value="null">
         <column name="Id" length="4" sql-type="int" not-null="true" unique="true" index="PK_Product"/>
         <generator class="native" />
      </id>

      <property name="Name" column="Name" type="String" length="50" not-null="true" />
      <property name="Description" column="Description" type="String" length="50" not-null="false" />

      <one-to-one name="Stock" class="Care.Inventory.Stock, Care.Inventory" cascade="all-delete-orphan" />
      <many-to-one name="Company" column="CompanyId" class="Care.Inventory.Company, Care.Inventory" not-null="true" />

   </class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="Care.Inventory.Stock, Care.Inventory" table="Stock">

      <id name="Id" column="Id" type="Int32">
         <generator class="foreign">
            <param name="property">Product</param>
         </generator>
      </id>

      <property name="Quantity" column="Quantity" type="Int32" length="4" not-null="true" />
      <one-to-one name="Product" class="Care.Inventory.Product, Care.Inventory" constrained="true" />

   </class>
</hibernate-mapping>

using System;
using System.Collections.Generic;

namespace Care.Inventory {

    public class Product {

        private int id;
        private string name;
        private string description;
        private Stock stock;
        private Company company;

        public Product() {
            name = String.Empty;
            description = String.Empty;

            company = new Company();
            stock = new Stock();
        }

        public virtual int Id {
            get { return id; }
            set { id = value; }
        }

        public virtual string Name {
            get { return name; }
            set { name = value; }
        }

        public virtual string Description {
            get { return description; }
            set { description = value; }
        }

        public virtual Stock Stock {
            get { return stock; }
            set {
                stock = value;
                stock.Product = this;
            }
        }

        public virtual Company Company {
            get { return company; }
            set { company = value; }
        }
    }
}

using System;
using System.Collections.Generic;

namespace Care.Inventory {

    public class Stock {

        private int id;
        private int quantity;
        private Product product;

        public virtual int Id {
            get { return id; }
            set { id = value; }
        }

        public virtual int Quantity {
            get { return quantity; }
            set { quantity = value; }
        }

        public virtual Product Product {
            get { return product; }
            set { product = value; }
        }
    }
}