Using checkbox list on ASP.NET MVC with Entity Framework 4.1 (telling the users about the third table is a leaky abstraction)

UPDATE August 2, 2011
The code is now efficient, movie.Genres.Add can just receive an stub object

[HttpPost]
public ActionResult Save(MovieInputViewModel input)
{
    using (var db = new TheMovieContext())
    {
        try
        {
            bool isNew = input.TheMovie.MovieId == 0;

            input.SelectedGenres = input.SelectedGenres ?? new List<int>();                    
            input.GenreSelections = db.Genres.AsNoTracking().OrderBy(x => x.GenreName).ToList();


            var movie = !isNew ? db.Movies.Find(input.TheMovie.MovieId) : input.TheMovie;                    

            if (isNew)
                db.Movies.Add(movie);
            else
            {
                db.Entry(movie).Property(x => x.Version).OriginalValue = input.TheMovie.Version;
                                        
                db.Entry(movie).State = EntityState.Modified;
            }


            movie.Genres = movie.Genres ?? new List<Genre>();
            movie.Genres.Clear();
            foreach (int g in input.SelectedGenres)
                movie.Genres.Add(db.LoadStub<Genre>(g));
            
            UpdateModel(movie, "TheMovie", includeProperties: null, excludeProperties: new string[] { "Version" });                   

            db.SaveChanges();

            
            input.TheMovie.Version = movie.Version;


            ModelState.Remove("TheMovie.MovieId");

            // No need to remove TheMovie.Version, ASP.NET MVC is not preserving the ModelState of variables with byte array type.
            // Hence, with byte array, the HiddenFor will always gets its value from the model, not from the ModelState
            // ModelState.Remove("TheMovie.Version"); 


            input.MessageToUser = input.MessageToUser + " " + string.Format("Saved. {0}", isNew ? "ID is " + input.TheMovie.MovieId : "");

        }
        catch (DbUpdateConcurrencyException)
        {
            ModelState.AddModelError("", 
                "The record you attempted to edit was already modified by another user since you last loaded it. Open the latest changes on this record");
        }
    }

    return View("Input", input);
}

We changed the Movie rowversion's attribute to Timestamp attribute(from ConcurrencyCheck attribute) so we will not need any work-around anymore(always setting one property to force-dirty the record)

public class Movie
{
    [Key]
    public virtual int MovieId { get; set; }
    
    [   Required, Display(Name="Title")
    ]   public virtual string MovieName { get; set; }
    
    [   Required, Display(Name="Description")
    ]   public virtual string MovieDescription { get; set; }
    
    [   Required, Display(Name="Year Released"), Range(1900,9999)
    ]   public virtual int? YearReleased { get; set; }
    
    [Timestamp]
    public virtual byte[] Version { get; set; }

    
    public virtual IList<Genre> Genres { get; set; }              
}

Here's the LoadStub extension method. If we could know how to get the primary key name of an entity, we will remove the primayKeyName property in code. For now, the convention-over configuration(using Entity name + ID as the default primary key) could prevent explicitly passing the primary key name.

Get the LoadStub code at http://www.ienablemuch.com/2011/08/entity-frameworks-nhibernate_02.html

---

Old post

Using checkboxes is a many-to-many design. In this post, I'll show you how to make the process of inserting to a third table a fairly automatic process with Entity Framework.

Most of the supporting logic is similar with http://www.ienablemuch.com/2011/07/using-checkbox-list-on-aspnet-mvc-with.html(NHibernate version)

The only difference is in the way the model is being saved.

[HttpPost]
public ActionResult Save(MovieInputViewModel input)
{
    using (var db = new TheMovieContext())
    {
        try
        {
            bool isNew = input.TheMovie.MovieId == 0;

            input.SelectedGenres = input.SelectedGenres ?? new List<int>();                    
            input.GenreSelections = db.Genres.AsNoTracking().OrderBy(x => x.GenreName).ToList();


            var movie = !isNew ? db.Movies.Find(input.TheMovie.MovieId) : input.TheMovie;

            if (isNew)
                db.Movies.Add(movie);
            else
            {
                db.Entry(movie).Property("Version").OriginalValue = input.TheMovie.Version;
                db.Entry(movie).State = System.Data.EntityState.Unchanged;


                // dirty this all the time even this is not changed by the user, 
                // so we have a simplified mechanism for concurrent update 
                // when the associated Genre(s) is modified
                db.Entry(movie).Property("MovieName").IsModified = true;
            }


            movie.Genres = movie.Genres ?? new List<Genre>();
            movie.Genres.Clear();
            foreach (int g in input.SelectedGenres)
            {
                // What is Entity Framework 4.1 analogous to NHibernate's session.Load<Genre>(g) ? My google-fu is failing me
                // db.Genres.Find(g) is not efficient
                movie.Genres.Add(db.Genres.Find(g));
                // Solution for db.Genres.Find inefficiency:
                // http://www.ienablemuch.com/2011/07/entity-framework-asnotracking-and.html
            }

            // http://www.joe-stevens.com/2010/02/17/asp-net-mvc-using-controller-updatemodel-when-using-a-viewmodel/
            // null will get all properties under TheMovie object, the Version is excluded                 
            UpdateModel(movie, "TheMovie", includeProperties: null, excludeProperties: new string[] { "Version" });



            db.SaveChanges();

            db.Entry(movie).Reload();
            input.TheMovie.Version = movie.Version;


            ModelState.Remove("TheMovie.MovieId");

            // No need to remove TheMovie.Version, ASP.NET MVC is not preserving the ModelState of variables with byte array type.
            // Hence, with byte array, the HiddenFor will always gets its value from the model, not from the ModelState
            // ModelState.Remove("TheMovie.Version"); 



            input.MessageToUser = string.Format("Saved. {0}", isNew ? "ID is " + input.TheMovie.MovieId : "");

        }
        catch (DbUpdateConcurrencyException)
        {
            ModelState.AddModelError("", 
                "The record you attempted to edit was already modified by another user since you last loaded it. Open the latest changes on this record");
        }
    }

    return View("Input", input);
}

The models. Note, the following models has a circular reference, this sort of thing is not possible on a platform like RDBMS. That's why platforms that can describe many-to-many scenario in an elegant manner and can be conveyed intuitively to users, is very appealing to developers who want to model the problem domain in terms of user's lingo, and fortunately one of those platforms is your favorite language(mine happens to be C#), C# don't have problems with circular references . Users don't know the fuss what's a third table is all about. Don't tell them about a third table, that will be a leaky abstraction. It's for you(developer) to infer if a third table is warranted; an example, a Genre happen to have many Movies, and a Movie can have many Genres too.

public class Genre
{
 [Key]
 public virtual int GenreId { get; set; }
 public virtual string GenreName { get; set; }

 public virtual IList<Movie> Movies { get; set; }
}


public class Movie
{
 [Key]
 public virtual int MovieId { get; set; }
 
 [   Required, Display(Name="Title")
 ]   public virtual string MovieName { get; set; }
 
 [   Required, Display(Name="Description")
 ]   public virtual string MovieDescription { get; set; }
 
 [   Required, Display(Name="Year Released"), Range(1900,9999)
 ]   public virtual int? YearReleased { get; set; }
 
 [ConcurrencyCheck]
 public virtual byte[] Version { get; set; }

 
 public virtual IList<Genre> Genres { get; set; }               
}

ORM mapping. We have three tables, namely Movie, Genre, and MovieAssocGenre.

public class TheMovieContext : DbContext
{
 public DbSet<Movie> Movies { get; set; }
 public DbSet<Genre> Genres { get; set; }


 protected override void OnModelCreating(DbModelBuilder modelBuilder)
 {           
  modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();

  modelBuilder.Entity<Movie>().HasMany(x => x.Genres).WithMany(x => x.Movies).Map(x =>
   {
    x.ToTable("MovieAssocGenre");
    x.MapLeftKey("MovieId");
    x.MapRightKey("GenreId");
   });

  modelBuilder.Entity<Genre>().HasMany(x => x.Movies).WithMany(x => x.Genres).Map(x =>
  {
   x.ToTable("MovieAssocGenre");
   x.MapLeftKey("GenreId");
   x.MapRightKey("MovieId");
  });
  
  
  base.OnModelCreating(modelBuilder);
 }


}

DDL:

create table Movie
(
MovieId int identity(1,1) not null primary key,
MovieName varchar(100) not null unique,
MovieDescription varchar(100) not null,
YearReleased int not null,
Version rowversion not null
);

create table Genre
(
GenreId int identity(1,1) not null primary key,
GenreName varchar(100) not null unique
);
 
create table MovieAssocGenre
(
MovieAssocGenreId int identity(1,1) not null primary key,
MovieId int not null references Movie(MovieId),
GenreId int not null references Genre(GenreId),
constraint uk_MovieAssocGenre unique(MovieId, GenreId)
);
  
insert into Genre(GenreName) values('Action'),('Comedy'),('Documentary'),('Romance'),('Sci-Fi'),('Thriller');

Get the complete code from http://code.google.com/p/ef-aspnet-mvc-checkbox-list-demo/downloads/list

Solution found on db.Genres.Find always hitting the database: http://www.ienablemuch.com/2011/07/entity-framework-asnotracking-and.html

Sample output