Repository pattern and one to many relationship

Question

I am creating an application in which I use Entity Framework. I have 2 classes with one-to-many relation .I decided to use a design pattern Repository as far as I know it is a good practice. My Interface:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace DataAccess.Repository
{
   public interface IRepository<T>
   {
       void Insert(T entity);
       void Delete(T entity);
       IQueryable<T> SearchFor(Expression<Func<T, bool>> predicate);
       IEnumerable<T> GetAll();
       T GetById(int id);
   }
}

My class

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
using DataAccess.Repository;

namespace DataAccess
{
    public class Repository<T> : IRepository<T>  where T : class
    {
        protected DbSet<T> DbSet;

        public Repository(DbContext datacontext)
        {
            //DbContext.Set Method (Type)
            //Returns a non-generic DbSet instance for access to entities of the given type in the context and the underlying store.
            DbSet = datacontext.Set<T>();
        }

        public void Insert(T entity)
        {
            DbSet.Add(entity);
        }

        public void Delete(T entity)
        {
            DbSet.Remove(entity);
        }

        public IQueryable<T> SearchFor(Expression<Func<T, bool>> predicate)
        {
            return DbSet.Where(predicate);
        }

        public IEnumerable<T> GetAll()
        {
            return DbSet;
        }

        public T GetById(int id)
        {
            return DbSet.Find(id);

        }
    }
}

And this is my two model classes

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Model
{
    public class Book
    {

        public int BookId { get; set; }
        public string Name { get; set; }
        public string Author { get; set; }
        public string Ganre { get; set; }
        public int Size { get; set; }
        public string Path { get; set; }

    }
}

and

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Model
{
    public  class Category{

        public Category()
        {
            Books = new List<Book>();
        }
        public int CategoryId { get; set; }
        public string CategoryName { get; set; }
        virtual public ICollection<Book> Books { get; set; } 
       }
}

But my question is, how to add the book to a category? This is an example of my implementation but book is not added to the category. But when I want to get all the books or all categories everything works fine.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.Entity;
using Model;
using DataAccess;
namespace TestDB
{
    class Program
    {
        static void Main(string[] args)
        {
            Database.SetInitializer(
                new DropCreateDatabaseIfModelChanges<BookShelfContext>());

            using (var db = new BookShelfContext())
            {
                var book = new Book
                {
                    Author = "Author Name",
                    Ganre = "Ganre",
                    Name = "Book Name",
                    Path = @"Path",
                    Size = 10
                };
                var category = new Category
                {
                    CategoryName = "Interesting"
                };


                var bookrepository = new Repository<Book>(db);
                var categoryrepository = new Repository<Category>(db);

                IEnumerable<Book> books = bookrepository.GetAll();
                IEnumerable<Category> categories = categoryrepository.GetAll();
                //get all books for example
                foreach (var b in books)
                {
                    Console.WriteLine(b.Name);
                }

            }
            Console.ReadKey();
        }
     }
 }

Thank you very much for your help. Have a nice day and less bugs)

Answer 1

Add the context to your Repository so that you can implement the SaveChanges method:

protected readonly DbContext context;

    public Repository(DbContext datacontext)
    {
        DbSet = datacontext.Set<T>();
        context = datacontext;
    }

    public void SaveChanges()
    {
        context.SaveChanges();
    }

Then in order to add a book to an existing BookCategory simply add the book to the Category's collection and save the category:

var categoryrepository = new Repository<Category>(db);
var myCategory = categoryrepository.GetById(1);
myCategory.Books.Add(book);
categoryrepository.SaveChanges();

Remember to call SaveChanges in order to persist the data in the database. EF is smart enough to notice that you added a child to the category and it will mark it as Added. On saving changes it will insert it to the database together with the foreign keys that it needs.