Linq To Sql Many-Many Join Table

Question

I am a somewhat experienced Rails developer and I thought I would try out ASP.NET's version of MVC. In doing so I also decided to try Linq->Sql...

I am a bit confused about the way Linq->Sql handles joins.

A trivial example of my schema is :

books:
id
title

categories:
id
name

books_categories:
book_id
category_id

Simply dragging these tables to the .dbml file doesn't seem to do it. I get a property on my Book class books_categories, what I expect is a property that I can iterate over and get Category classes directly.

Right now I have to do something that feels very wrong

        foreach (books_categories bc in book.books_categories)
        {
            category_names.Add(bc.Category.category.Trim());
        }

[In Response to Accepted answer]
I grudgingly accepted the answer of "write your own glue code". After continuing my research of Linq->Sql I discovered that it is apparently slowly being let go in favor of the (more powereful, IMO) Entity Framework. EF still allows one to use LINQ for queries and does a decent job of figuring out relationships like Ruby's ActiveRecord.

Answer 1

I don't think your expected result is supported in Linq to Sql.

What you're doing may feel wrong, but I think that's one way to work around that limitation of L2S.

Man, i've really got to get into Rails...

Answer 2

Use a partial class implementation for Book and add appropriate methods for categories and their properties. Have the properties front-end the Books_Categories property (you can make this have private visibility to force implementation through your Categories property).

public partial class Books
{
    public IEnumerable<string> CategoryNames
    {
       get
       {  
            return this.Books_Categories
                       .Select( bc => bc.Category.category.Trim() );
       }
    }

    public void AddCategory( Category category )
    {
       this.Books_Categories.Add( new Book_Category
                                  {
                                      Category = category,
                                      Book = this
                                  } );
    }

    public void RemoveCategory( Category category )
    {
       var bc = this.Book_Categories
                    .Where( c => c.Category == category )
                    .SingleOrDefault();
       this.Books_Categories.Remove( bc );
    }
}

Obviously, you'll need to add some error/bounds checking, etc. but you get the idea.

I'll grant you this is not ideal, but at least you have the flexibility to determine how it works.

Answer 3

Many to many mappings are explicitly supported in the Entity Framework, but not in LINQ to SQL. You can also use third-party ORMs such as NHibernate.

Answer 4

What you can do if you want to create a book and directly want to add a category to it is: in your view:

        <p>
            <label for="CategorySelect">Category:</label>
            <%= Html.ListBox("CategorySelect") %>
            <%= Html.ValidationMessage("CategorySelect", "*")%>
        </p>

in your bookscontroller:

        public ActionResult New()
    {
            var data = _entities.Categories.ToList();
        ViewData["CategorySelect"] = new MultiSelectList(data, "Id", "Name");
}

        [AcceptVerbs(HttpVerbs.Post)]
    public ActionResult New([Bind(Exclude = "Id")] Book Booknew)
    {
        IEnumerable<int> selectedCategories = Request["CategorySelect"].Split(new Char[] { ',' }).Select(idStr => int.Parse(idStr));

        if (!ModelState.IsValid)
            return View();

        try {
            foreach(var item in selectedCategories){
                BooksCategories bc = new BooksCategories();
                bc.Book = Booknew;
                bc.CategoryId = item;
                _entities.BooksCategories.InsertOnSubmit(bc);
            }
            _entities.Books.InsertOnSubmit(Booknew);
            _entities.SubmitChanges();