public class Book
    {
        private IList<Color> _colors;
        private string _name;
        private int _id;

        public Book()
        {
            _colors = new List<Color>();
        }
        public virtual string Name
        {
            get { return _name; }
            set { _name = value; }
        }

        public virtual IList<Color> Colors
        {
            get { return _colors; }
            set { _colors = value; }
        }

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

    public class Color
    {
        private int _id;
        private string _name;

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

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

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHibernate.Probing" assembly="NHibernate.Probing">

	<class name="Book" table="books">
		<id name="Id" column="Id">
			<generator class="native" />
		</id>
		<property name="Name" type="string" column="name" />
		<bag name="Colors" cascade="all">
			<key column="BookId"/>
			<one-to-many class="Color"/>
		</bag>
	</class>

	<class name="Color" table="colors">
		<id column="Id" name="Id">
			<generator class="native" />
		</id>
		<property name="Name" type="string" column="name" />
	</class>

</hibernate-mapping>

 [TestFixture]
    public class NHibernateFixture
    {
        #region Setup/Teardown

        [SetUp]
        public void SetUp()
        {
            _config = new Configuration();
            _config.Configure("hibernate.cfg.xml");

            _sessionFactory = _config.BuildSessionFactory();
            new SchemaExport(_config).Drop(false, true);
            new SchemaExport(_config).Execute(false, true, false);
        }

        #endregion

        private ISessionFactory _sessionFactory;
        private Configuration _config;

        [Test]
        public void Test()
        {
            using (ISession session = _sessionFactory.OpenSession())
            {
                Book book = new Book();

                book.Colors.Add(new Color());

                session.SaveOrUpdate(book);
                session.Flush();
                session.Clear();

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

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

    public class Book
    {
        private IList<Color> _colors;
        private string _name;
        private int _id;

        public Book()
        {
            _colors = new List<Color>();
        }
        public virtual string Name
        {
            get { return _name; }
            set { _name = value; }
        }

        public virtual IList<Color> Colors
        {
            get { return _colors; }
            set { _colors = value; }
        }

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

    public class Color
    {
        private int _id;
        private string _name;
        private Book _book;

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

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

        public virtual Book Book
        {
            get { return _book; }
            set { _book = value; }
        }

<?xml version="1.0" encoding="utf-8"?>

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

	<class name="Book" table="books">
		<id name="Id" column="Id">
			<generator class="native" />
		</id>
		<property name="Name" type="string" column="name" />
		<bag name="Colors" cascade="all-delete-orphan" inverse="true">
			<key column="BookId"/>
			<one-to-many class="Color" />
		</bag>
	</class>

	<class name="Color" table="colors">
		<id column="Id" name="Id">
			<generator class="native" />
		</id>
		<property name="Name" type="string" column="name" />
		<many-to-one name="Book" class="Book" cascade="all">
			<column name="idColor" />
		</many-to-one>
	</class>

</hibernate-mapping>

    [TestFixture]
    public class NHibernateFixture
    {
        #region Setup/Teardown

        [SetUp]
        public void SetUp()
        {
            _config = new Configuration();
            _config.Configure("hibernate.cfg.xml");

            _sessionFactory = _config.BuildSessionFactory();
            new SchemaExport(_config).Drop(false, true);
            new SchemaExport(_config).Execute(true, true, false);
        }

        #endregion

        private ISessionFactory _sessionFactory;
        private Configuration _config;

        [Test]
        public void Test2()
        {
            using (ISession session = _sessionFactory.OpenSession())
            {
                Book book = new Book();

                book.Colors.Add(new Color());
                book.Colors.Add(new Color());
                book.Colors[0].Book = book;
                book.Colors[1].Book = book;

                session.SaveOrUpdate(book);
                session.Flush();
                session.Clear();

                Color color = session.Get<Color>(book.Colors[1].Id);
                session.Delete(color);
                session.Flush();
            }
        }

 create table books (
        Id INT IDENTITY NOT NULL,
       name NVARCHAR(255) null,
       primary key (Id)
    )

    create table colors (
        Id INT IDENTITY NOT NULL,
       name NVARCHAR(255) null,
       idColor INT null,
       BookId INT null,
       primary key (Id)
    )

    alter table colors 
        add constraint FK5E59605F263D6D63 
        foreign key (idColor) 
        references books

    alter table colors 
        add constraint FK5E59605FDBD2D929 
        foreign key (BookId) 
        references books
NHibernate: INSERT INTO books (name) VALUES (@p0); select SCOPE_IDENTITY();@p0 = NULL
NHibernate: INSERT INTO colors (name, idColor) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = NULL, @p1 = 1
NHibernate: INSERT INTO colors (name, idColor) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = NULL, @p1 = 1
NHibernate: SELECT color0_.Id as Id1_1_, color0_.name as name1_1_, color0_.idColor as idColor1_1_, book1_.Id as Id0_0_, book1_.name as name0_0_ FROM colors color0_ left outer join books book1_ on color0_.idColor=book1_.Id WHERE color0_.Id=@p0;@p0 = 2
NHibernate: SELECT colors0_.BookId as BookId2_, colors0_.Id as Id2_, colors0_.Id as Id1_1_, colors0_.name as name1_1_, colors0_.idColor as idColor1_1_, book1_.Id as Id0_0_, book1_.name as name0_0_ FROM colors colors0_ left outer join books book1_ on colors0_.idColor=book1_.Id WHERE colors0_.BookId=@p0;@p0 = 1
NHibernate: DELETE FROM colors WHERE Id = @p0;@p0 = 2
NHibernate: DELETE FROM books WHERE Id = @p0;@p0 = 1
18:20:38,455 ERROR [   7] AbstractBatcher [(null)]- Could not execute command: DELETE FROM books WHERE Id = @p0
System.Data.SqlClient.SqlException: DELETE statement conflicted with COLUMN REFERENCE constraint 'FK5E59605F263D6D63'. The conflict occurred in database 'Integration_Tests', table 'colors', column 'idColor'.
The statement has been terminated.