Bind MVC DropDownlistFor () for Navigation Model Property

I am writing a simple learning application for my C # MVC class and our professor is using us in a terrible memory system using session and lists because he hasn't started teaching entity framework yet

the problem i'm running into is that when i try to bind the dropdown to my model it gives me an unexpected result.

im code used in view:

<div class="editor-label">
    @Html.LabelFor(model => model.Author)
</div>
<div class="editor-field">
    @Html.DropDownListFor(m => m.Author, new SelectList(new Lab2Repository().GetAuthors(), "Id", "Name"), "---- Select Category ----")
    @Html.ValidationMessageFor(m=>m.Author)
</div>
<div class="editor-label">
    @Html.LabelFor(m=>m.Category)
</div>
<div class="editor-field">
    @Html.DropDownListFor(m => m.Category, new SelectList(new Lab2Repository().GetCategories(), "Id", "Name"), "---- Select Category ----")
    @Html.ValidationMessageFor(m => m.Category)

</div>

      

        
        
        
      

    

and when i run this code it gives validation error no matter what is selected

Repository code:

using System;
using System.Linq;
using System.Collections.Generic;
using Int422.Lab2.Data;
using System.Web;
using Int422.Lab2.Models;


public class Lab2Repository
{
    public Lab2Context Data { get; set; }
    public Lab2Repository()
    {
        Data = (Lab2Context)HttpContext.Current.Application["State"];
    }

    #region Retrieval

    public IEnumerable<Book> GetBooks()
    {
        return Data.Books;
    }
    public IEnumerable<Author> GetAuthors()
    {
        return Data.Authors.Select(obj => new Author(obj.Name) 
        { 
            Id = obj.Id, 
            Books = GetBooks().Where(b=>b.Author.Id == obj.Id).ToList() 
        });
    }

    public IEnumerable<Category> GetCategories()
    {
        return Data.Categories.Select(obj => new Category(obj.Name, obj.Description) 
        {  
            Id = obj.Id, 
            Books = GetBooks().Where(b=>b.Category.Id == obj.Id).ToList(),
        });


    } 
    #endregion
    #region Singles
    public Author GetAuthor(int id)
    {
        return GetAuthors().Where(obj => obj.Id == id).Single();
    }

    public Category GetCategory(int id)
    {
        return GetCategories().Where(obj => obj.Id == id).Single();
    }

    public Book GetBook(int id)
    {
        return Data.Books.Where(obj => obj.Id == id).Single();
    }
    #endregion
    #region Edit
    public void Edit(int id, Category rec)
    {
        if (id != rec.Id)
            throw new InvalidOperationException("Id Mismatch during edit process");

        int index = Data.Categories.IndexOf(Data.Categories.Single(p => p.Id == id));
        if (index == -1)
            throw new InvalidOperationException("Record not found no record to edit");
        Data.Categories[index] = rec;
        SaveChanges();

    }
    public void Edit(int id, Book rec)
    {
        if (id != rec.Id)
            throw new InvalidOperationException("Id Mismatch during edit process");

        int index = Data.Books.IndexOf(Data.Books.Single(p => p.Id == id));
        if (index == -1)
            throw new InvalidOperationException("Record not found no record to edit");
        Data.Books[index] = rec;
        SaveChanges();
    }


    public void Edit(int id, Author rec)
    {
        if (id != rec.Id)
            throw new InvalidOperationException("Id Mismatch during edit process");

        int index = Data.Authors.IndexOf(Data.Authors.Single(p => p.Id == id));
        if (index == -1)
            throw new InvalidOperationException(String.Format("Record id {0} not found", id));
        Data.Authors[index] = rec;
        SaveChanges();

    }
    #endregion
    #region Create
    //
    public int Create(Category m)
    {
        m.Id = Data.Categories.Max(o => o.Id) + 1;
        Data.Categories.Add(m);
        SaveChanges();
        return m.Id;

    }

    //adds new author to the collection returns the id 
    public int Create(Author m)
    {
        m.Id = Data.Authors.Max(o => o.Id) + 1;
        Data.Authors.Add(m);
        SaveChanges();
        return m.Id;
    }

    public int Create(Book m)
    {
        m.Id = Data.Authors.Max(o => o.Id) + 1;
        Data.Books.Add(m);
        SaveChanges();
        return m.Id;
    }
    #endregion
    #region Delete

    public void Delete(Author rec)
    {
        Data.Authors.Remove(Data.Authors.Single(o => o.Id == rec.Id));
        SaveChanges();
    }

    public void Delete(Category m)
    {
        Data.Categories.Remove(Data.Categories.Single(o => o.Id == m.Id));
        SaveChanges();
    }

    public void Delete(Book m)
    {
        Data.Books.Remove(Data.Books.Single(o => o.Id == m.Id));
        SaveChanges();
    }





    #endregion
    #region Session Persistance

    public void SaveChanges()
    {
        //save to session 
        HttpContext.Current.Application["State"] = Data;
        //refresh repo state from session 
        Data = (Lab2Context)HttpContext.Current.Application["State "];
    }

    #endregion




}

      

        
        
        
      

    

Models:

    public class Author
    {
        public Author(string name)
        {
            Name = name;
        }
        public Author()
        {
            Books = new List<Book>();
        }
        public int Id { get; set; }
        public string Name { get; set; }
        public virtual ICollection<Book> Books { get; set; }

    }

public class Category
    {

        public Category(string name, string desc)
        {
            Name = name;
            Description = desc;
            Books = new List<Book>();
        }
        public Category()
        {

        }
        public int Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public virtual ICollection<Book> Books  { get; set; }

    }

public class Book
    {
        public Book(string title, Author author, Category category, double retailPrice, double cost)
        {
            Title = title;
            Author = author;
            Category = category;
            RetailPrice = retailPrice;
            Cost = cost; 
        }
        public Book()
        {

        }

        public int Id { get; set; }
        public Category Category { get; set; }
        public Author Author { get; set; }
        public string Title { get; set; }
        public double RetailPrice { get; set; }
        public double Cost { get; set; }

    }

      

        
        
        
      

    

Controller:

public partial class BooksController : Controller
    {
        private Lab2Repository repo = new Lab2Repository();

        //
        // GET: /Books/

        public virtual ViewResult Index()
        {
            return View(repo.GetBooks());
        }

        //
        // GET: /Books/Details/5
        [GET("Books/{id:int}")]
        public virtual ActionResult Details(int id)
        {
            Book book = repo.GetBook(id);
            return View(book);
        }

        //
        // GET: /Books/Create

        public virtual ActionResult Create()
        {
            return View();
        } 

        //
        // POST: /Books/Create

        [HttpPost]
        public virtual ActionResult Create(Book book)
        {
            if (!ModelState.IsValid)
                return View(book);

            repo.Create(book);

            return RedirectToAction(MVC.Books.Index());

        }

        //
        // GET: /Books/Edit/5

        public virtual ActionResult Edit(int id)
        {
            Book book = repo.GetBook(id);
            return View(book);
        }

        //
        // POST: /Books/Edit/5

        [HttpPost]
        public virtual ActionResult Edit(int id, Book book)
        {
            if (!ModelState.IsValid)
                return View(book);

            repo.Edit(id, book);

            return RedirectToAction(MVC.Books.Details(id));
        }

        //
        // GET: /Books/Delete/5

        public virtual ActionResult Delete(int id)
        {
            Book book = repo.GetBook(id);
            return View(book);
        }

        //
        // POST: /Books/Delete/5

        [HttpPost, ActionName("Delete")]
        public virtual ActionResult DeleteConfirmed(int id)
        {
            Book book = repo.GetBook(id);
            repo.Delete(book);

            return RedirectToAction("Index");
        }


    }

      

        
        
        
      

    

Initializer:

public class Lab2StoreInitializer
{
public void Seed(Lab2Context c)
{

    //add categories 
    var cat_Programming = new Category("Programming", "Books about writing computer code and developing software") { Id = 1 };
    var cat_Politics = new Category("Politics", "books with politically charged subjects") { Id = 2 };
    var cat_NonFiction = new Category("Non-Fiction", "Books about real life events") { Id = 3 };


    c.Categories.Add(cat_Programming);
    c.Categories.Add(cat_Politics);
    c.Categories.Add(cat_NonFiction);


    //add authors 
    var noamChomsky = new Author("Noam Chomsky") { Id = 1};
    var howardZinn = new Author("Howard Zinn") { Id = 2 };
    var scottHanselman = new Author("Scott Hanselman") { Id = 3 };
    var philHaack = new Author("Phil Haack") { Id = 4 };
    var danielleSteele = new Author("Danielle Steele") { Id = 5 };
    var johnSkeet = new Author("John Skeet") { Id = 6 };
    var johnResig = new Author("John Resig") { Id = 7 };
    var scottKlein = new Author("Scott Klein") { Id = 8 };


    c.Authors.Add(noamChomsky);
    c.Authors.Add(howardZinn);
    c.Authors.Add(scottHanselman);
    c.Authors.Add(philHaack);
    c.Authors.Add(danielleSteele);
    c.Authors.Add(johnSkeet);
    c.Authors.Add(johnResig);
    c.Authors.Add(scottKlein);

    // add books 

    c.Books.Add(new Book("Manufacturing Consent", noamChomsky, cat_Politics, 16.57, 12.00) { Id = 1 });
    c.Books.Add(new Book("The Zinn Reader", howardZinn, cat_Politics, 35.75, 30.00) { Id = 2 });
    c.Books.Add(new Book("C# In Depth", johnSkeet, cat_Programming, 29.99, 25.00) { Id = 3 });
    c.Books.Add(new Book("Professional ASP.NET MVC 2", scottHanselman, cat_Programming, 34.99, 30.00) { Id = 4 });
    c.Books.Add(new Book("Professional MVC 4", philHaack, cat_Programming, 49.99, 45.00) { Id = 5 });
    c.Books.Add(new Book("His Bright Light: The Story of Nick Traina", danielleSteele, cat_NonFiction, 19.99, 14.00) { Id = 6 });
    c.Books.Add(new Book("Secrets of the JavaScript Ninja", johnResig, cat_Programming, 34.99, 29.99) { Id = 7 });
    c.Books.Add(new Book("Professional LINQ", scottKlein, cat_Programming, 39.99, 35.00) { Id = 8 });
    c.Books.Add(new Book("jQuery in Action", johnResig, cat_Programming, 49.99, 43.00) { Id = 9 });


    HttpContext.Current.Application["State"] = c;
    //base.Seed(c);
}
}