EF Many-to-Many Self-Reference 2

//Item.cs
    public class Item
    {
        public Item()
        {
            Composites = new HashSet<ItemItem>();
            Components = new HashSet<ItemItem>();
        }

        public int ItemId { get; set; }

        public string Name { get; set; }

        public int Cost { get; set; }

        public virtual ICollection<ItemItem> Composites { get; set; }

        public virtual ICollection<ItemItem> Components { get; set; }

        public void AddComponent(MyContext context, Item item, [Range(1, Int32.MaxValue)] int quantity = 1)
        {
            var itemRelation = Components.FirstOrDefault(c => c.Composite == this);
            if (itemRelation == null)
            {
                itemRelation = new ItemItem() { Composite = this, Component = item, Quantity = quantity };
                this.Components.Add(itemRelation);
                item.Composites.Add(itemRelation);
            }
            else
            {
                itemRelation.Quantity += quantity;
            }
        }

        public void AddComposite(MyContext context, Item item, [Range(1, Int32.MaxValue)] int quantity = 1)
        {
            var itemRelation = Composites.FirstOrDefault(c => c.Component == this);
            if (itemRelation == null)
            {
                itemRelation = new ItemItem() { Component = this, Composite = item, Quantity = quantity };
                this.Composites.Add(itemRelation);
                item.Components.Add(itemRelation);
            }
            else
            {
                itemRelation.Quantity += quantity;
            }
        }
    }

//ItemItem.cs
    public class ItemItem
    {
        public ItemItem()
        {

        }

        [Key]
        public int RelationId { get; set; }

        public int Quantity { get; set; }

        public virtual Item Composite { get; set; }

        public virtual Item Component { get; set; }
    }

//MyContext.cs
    public class MyContext : DbContext
    {
        public MyContext()
            :base("DbConnection")
        { }

        public DbSet<Item> Items { get; set; }

        public DbSet<ItemItem> ItemItems { get; set; }
    }

//MainWindow.xaml.cs
public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            AppDomain.CurrentDomain.SetData("DataDirectory", AppDomain.CurrentDomain.BaseDirectory);
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            Database.SetInitializer<MyContext>(new CreateDatabaseIfNotExists<MyContext>());

            using (MyContext context = new MyContext())
            {

                Item i1 = new Item() { Name = "Motherboard", Cost = 500 };
                Item i2 = new Item() { Name = "Network Adapter", Cost = 150 };
                Item i3 = new Item() { Name = "System Unit", Cost = 0 };

                context.Items.Add(i1);
                context.Items.Add(i2);
                context.Items.Add(i3);

                context.SaveChanges();

                i3.AddComponent(context, i1);
                i3.AddComponent(context, i2);

                context.SaveChanges();
            }
        }
    }