EF 4.1 RC Many to many relationship in EF CF

Question

I have two entities with many to many relationship like this:

class author
{
  public int AuthorID{get;set;}
  public string Name{get;set;}
  public virtual ICollection<book> books{get;set;}
}
class book
{
  public int BookID{get;set;}
  public string Name{get;set;}
 public virtual ICollection<author> authors{get;set;}

}

and I have an intermediate table named Bookauthor defined like this:

  BookAuthor: int
  int AuthorID
  int BookID

How to map this using FluentAPI

Thanks!!

Answer 1

I don't think EF allows you to have a separate Id in many-to-many junction tables.

The alternative is just mapping BookAuthor as an entity.

On a side note, NHibernate supports this construct using an idbag

Answer 2

This was problematic with EDMX but with EF 4.1 fluent API you can map it:

modelBuilder.Entity<book>()
            .HasMany(b => b.authors)
            .WithMany(a => a.books)
            .Map(m => m.MapLeftKey("BookID")
                   .MapRightKey("AuthorID")
                   .ToTable("BookAuthor"));

As you can see I don't map BookAuthor column. That column is unknown to EF and must be auto incremented.

This obviously can't work with a Code-first approach but only if you use Fluent API against existing database.