OnModelCreating

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

            modelBuilder.Entity<ItemItem>().HasOptional(u => u.Component)
                .WithMany(a => a.Composites)
                .HasForeignKey(u => u.ComponentId);

            modelBuilder.Entity<ItemItem>().HasOptional(u => u.Composite)
                .WithMany(a => a.Components)
                .HasForeignKey(u => u.CompositeId);
        }