Question

I have following Classes and interfaces..

 public interface ITaggable
    {
         ICollection<Tag> Tags { get; set; }
    }

 public class Book :ITaggable
    {
        [DatabaseGenerated(DatabaseGenerationOption.Identity)] 
        public Guid BookId { get;set; }
        public string Title { get; set; }
        public string Author { get; set; }
        public virtual ICollection<Tag> Tags {get; set;}
    }

public class Pen:ITaggable
    {
        [DatabaseGenerated(DatabaseGenerationOption.Identity)]
        public Guid PenId { get; set; }
        public string Color { get; set; }
        public virtual ICollection<Tag> Tags {get; set;}
    }

public class Tag
    {
        [DatabaseGenerated(DatabaseGenerationOption.Identity)] 
        public Guid TagId { get; set; }
        public string Name { get; set; }
        public virtual ICollection<ITaggable> Items { get; set; }
    }

For the above model it generates the following table structure

Book –>
BookId , Title , Author

Pen –>
PenId , Color

Tag –>
TagId , Name ,BookBookId , PenPenId

And when i insert the following data

            Tag tag = new Tag();
            tag.Name = "Stationary";

            Book b1 = new Book();
            b1.Title = "Head first c#";
            b1.Author = "Betty";
            b1.Tags = new List<Tag>() { tag };

            Book b2 = new Book();
            b2.Title = "Head first Java";
            b2.Author = "Katty";
            b2.Tags = new List<Tag>() { tag };

            Pen p = new Pen();
            p.Color = "Red";
            p.Tags = new List<Tag>() { tag };

            context.Books.Add(b1);
            context.Books.Add(b2);
            context.Pens.Add(p);
            context.SaveChanges();

It does not insert the Tag data for the second book

What i want to accomplish is that i want to implement a three table tagging system
shown here adhering to my class structure

Answer 1

Yes it will not store the tag info for one of the books because your tag instance can be associated only with single book. That would require your many-to-many relation between Book and Tag but your relation is one-to-many. Relation between Pen and Tag is also one-to-many. That is clearly visible by foreign keys in Tag table.

The problem is that ICollection<ITaggable> Items is skipped by EF code first – code first doesn’t work with interfaces. You must define your Tag as:

public class Tag
{
    [DatabaseGenerated(DatabaseGenerationOption.Identity)] 
    public Guid TagId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Book> Books { get; set; }
    public virtual ICollection<Pen> Pens { get; set; }
}

This will map many-to-many between Book and Tag and many-to-many between Pen and Tag. If you want also collection of ITaggable you can expose another property concatenating Books and Pens.

If you really want one-to-many relation then you cannot expect that tag will be associated with multiple books and in such case your Tag entity should look like:

public class Tag
{
    [DatabaseGenerated(DatabaseGenerationOption.Identity)] 
    public Guid TagId { get; set; }
    public string Name { get; set; }
    public virtual Book Book { get; set; }
    public virtual Pen Pen { get; set; }
}

Again you can create Items as computed property. Any combination of relation should be obvious from these examples.

Edit:

If you don’t want to expose navigation properties in Tag you must use fluent-api:

public class Context : DbContext
{
    public DbSet<Book> Books { get; set; }
    public DbSet<Pen> Pens { get; set; }
    public DbSet<Tag> Tags { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Book>()
                    .HasMany(b => b.Tags)
                    .WithMany();

        modelBuilder.Entity<Pen>()
                    .HasMany(p => p.Tags)
                    .WithMany();
    }
}

Further fluent api reference can be found on ADO.NET team blog but it is not up to data (it is for CTP5).