Fluent NHibernate - HasManyToMany NHibernate.MappingException: Repeated column in mapping for collection

Question

I'm a NHibernate novice trying to configure an existing database with Fluent NHibernate. The problem is with a many-to-many mapping, in this example represented by libraries and books. I guess this should be really basic stuff, but I get the following exception:

FluentNHibernate.Cfg.FluentConfigurationException: An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.

 ---> NHibernate.MappingException: Repeated column in mapping for collection: MvcNhibernatePoc.Models.Book.Libraries column: BookId

The structure of the database should not be changed and looks something like:

Table **Book**
BookId (int)
BookName (varchar(255))

Table **Library**
LibraryId (int)
LibraryName (varchar(255))

Table **Book_Library**
Id (int)
BookId (int)
LibraryId (int)

Based on this I have created the following domain classes:

public class Library
    {
        public virtual int LibraryId { get; set; }
        public virtual string Name { get; set; }

        public virtual IList<Book> Books { get; set; }

        public Library()
        {
            Books = new List<Book>();
        }
    }


public class Book
{
    public virtual int BookId { get; set; }
    public virtual string Name { get; set; }

    public virtual IList<Library> Libraries { get; set; }

    public Book()
    {
        Libraries = new List<Library>();
    }
}

Mapping:

public class LibraryMap : ClassMap<Library>
{
    public LibraryMap()
    {
        Table("Library");
        Id(l => l.LibraryId).Column("LibraryId");
        Map(l => l.Name).Column("LibraryName");

        HasManyToMany<Book>(l => l.Books)
            .Table("Book_Library")
            .ParentKeyColumn("LibraryId")
            .ChildKeyColumn("LibraryId")
            .Cascade.SaveUpdate();
    }
}

public class BookMap : ClassMap<Book>
{
    public BookMap()
    {
        Table("Book");
        Id(b => b.BookId).Column("BookId");
        Map(b => b.Name).Column("BookName");

        HasManyToMany<Library>(b => b.Libraries)
            .Table("Book_Library")
            .ParentKeyColumn("BookId")
            .ChildKeyColumn("BookId")
            .Cascade.SaveUpdate()
            .Inverse();
    }
}

Fluent configuration:

Fluently.Configure()
            .Database(MsSqlConfiguration.MsSql2008.ConnectionString(conString).ShowSql)
            .Mappings(m => m.FluentMappings
            .AddFromAssemblyOf<Library>())
            .BuildSessionFactory();

And finally my failing testcode:

var library = new Library { LibraryId = 1, Name = "Alexandria library"};
var book = new Book { BookId = 1, Name = "Pyramids for dummies" };

library.Books.Add(book);
book.Libraries.Add(library);

using (var session = NHibernateHelper.OpenSession())
{
    using (var transaction = session.BeginTransaction())
    {
        session.Save(library);
        session.Flush();

        transaction.Commit();

        Console.WriteLine("Saved library " + library.Name);
    }
}

Answer 1

        .ParentKeyColumn("BookId")
        .ChildKeyColumn("BookId")

        .ParentKeyColumn("LibraryId")
        .ChildKeyColumn("LibraryId")

should be

        // BookMap
        .ParentKeyColumn("BookId")
        .ChildKeyColumn("LibraryId")

        // LibraryMap
        .ParentKeyColumn("LibraryId")
        .ChildKeyColumn("BookId")