public class Book
{
    public virtual string Name { get; set; }
    public virtual Color Color { get; set; }
}

public class Color
{
    public virtual string Name { get; set; }
    public virtual IList<Book> Books { get; set; }
}

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHPoligon" assembly="NHPoligon" default-lazy="false">

  <class name="Book" table="books" >
    <id column="Id" type="int">
      <generator class="native"/>
    </id>

    <property name="Name" type="string" column="name"/>

    <many-to-one name="Color" class="Color" cascade="all-delete-orphan"  >
      <column name="idColor"/>
    </many-to-one>

  </class>
  
</hibernate-mapping>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHPoligon" assembly="NHPoligon" default-lazy="false">

  <class name="Color" table="colors">

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

    <property name="Name" type="string" column="name"/>

    <bag name="Books" inverse="true">
      <key column="Id"/>
      <one-to-many class="Book"/>  
    </bag>
    
  </class>

</hibernate-mapping>

using (NHibernate.ISession session = SessionManager.OpenSession())
{
    Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();
    
    session.Delete(book);

    session.Flush();
}

NHibernate: DELETE FROM books WHERE Id = @p0;@p0 = 26
NHibernate: DELETE FROM colors WHERE Id = @p0;@p0 = 10

Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();

SELECT this_.Id as Id0_1_, 
       this_.name as name0_1_, 
       this_.idColor as idColor0_1_, 
       color2_.Id as Id1_0_, 
       color2_.name as name1_0_ 
FROM books this_ 
left outer join colors color2_ on this_.idColor=color2_.Id 
WHERE this_.name = @p0;@p0 = 'First book'


SELECT books0_.Id as Id2_, 
       books0_.Id as Id0_1_, 
       books0_.name as name0_1_, 
       books0_.idColor as idColor0_1_, 
       color1_.Id as Id1_0_, 
       color1_.name as name1_0_ 
FROM books books0_ 
left outer join colors color1_ on books0_.idColor=color1_.Id 
WHERE books0_.Id=@p0;@p0 =  10 

session.Delete(book);
session.Flush();

DELETE FROM books WHERE Id = @p0;@p0 =  30 
DELETE FROM colors WHERE Id = @p0;@p0 =  10 

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHPoligon" assembly="NHPoligon">

  <class name="Color" table="colors">

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

    <property name="Name" type="string" column="name"/>

    <bag name="Books" inverse="true">
      <key column="Id"/>
      <one-to-many class="Book"/>  
    </bag>
    
  </class>

</hibernate-mapping>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHPoligon" assembly="NHPoligon">

  <class name="Book" table="books" >
    <id column="Id" type="int">
      <generator class="native"/>
    </id>

    <property name="Name" type="string" column="name"/>

    <many-to-one name="Color" class="Color" cascade="all-delete-orphan"  >
      <column name="idColor"/>
    </many-to-one>

  </class>
  
</hibernate-mapping>

Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();

SELECT this_.Id as Id0_0_, 
           this_.name as name0_0_, 
           this_.idColor as idColor0_0_ 
FROM books this_ 
WHERE this_.name = @p0;@p0 = 'First book'

session.Delete(book);

SELECT color0_.Id as Id1_0_, 
           color0_.name as name1_0_ 
FROM colors color0_ 
WHERE color0_.Id=@p0;@p0 =  10 

session.Flush();

DELETE FROM books WHERE Id = @p0;@p0 =  32 
DELETE FROM colors WHERE Id = @p0;@p0 =  10 

NHibernate.Exceptions.GenericADOException was unhandled
  Message="could not delete: [NHPoligon.Color#10][SQL: DELETE FROM colors WHERE Id = ?]"
  Source="NHibernate"
  SqlString="DELETE FROM colors WHERE Id = ?"
Message="The DELETE statement conflicted with the REFERENCE constraint \"FK_books_colors\". The conflict 
occurred in database \"NHPoligon\", table \"dbo.books\", column 'idColor'.\r\nThe statement has been terminated."

<many-to-one name="Color" class="Color" cascade="all-delete-orphan">

using (NHibernate.ISession session = SessionManager.OpenSession())
{
    Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();

    //Меняю цвет на новый, а старый остается без владельца
    book.Color = new Color() { Name = "Фиолетовый" };
    book.Color.Books.Add(book);

    session.Save(book);
    session.Flush();
}

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHPoligon" assembly="NHPoligon">

  <class name="Color" table="colors">

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

    <property name="Name" type="string" column="name"/>

  </class>

</hibernate-mapping>

public class Color
{
    public virtual string Name { get; set; }
}

using (NHibernate.ISession session = SessionManager.OpenSession())
{
    Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();

    book.Color = new Color(){Name = "Фиолетовый-3"};

    session.Save(book);
    session.Flush();
}

Book book = (Book)session.CreateCriteria(typeof(Book)).Add(Expression.Eq("Name", "First book")).UniqueResult();

SELECT this_.Id as Id0_0_, this_.name as name0_0_, this_.idColor as idColor0_0_ 
FROM books this_ 
WHERE this_.name = @p0;@p0 = 'First book'

book.Color = new Colorr() { Name = "Фиолетовый-5" };
session.Save(book);
session.Flush();

INSERT INTO colors (name) 
VALUES (@p0); 

select SCOPE_IDENTITY();@p0= 'Фиолетовый-5'

UPDATE books 
SET name = @p0, idColor = @p1 
WHERE Id = @p2;@p0 = 'First book', @p1 =  14 , @p2 =  33 

session.Clear();

Book book2Compare = session.Get<Book>(book.Id);

SELECT book0_.Id as Id0_0_, book0_.name as name0_0_, book0_.idColor as idColor0_0_ 
FROM books book0_ 
WHERE book0_.Id=@p0;@p0 =  33 

book2Compare.Color = null;

session.SaveOrUpdate(book2Compare);
session.Flush();

UPDATE books SET name = @p0, idColor = @p1 WHERE Id = @p2;@p0 = 'First book', @p1 = NULL, @p2 =  33