Using Entity inside AggregateRoot - NCQRS

gistfile1.cs

    public class Order : AggregateRoot
    {
        private readonly List<OrderItem> orderItems = new List<OrderItem>();

        public void UpdateOrderItem(Guid orderItemId, int quantity)
        {
            var orderItem = orderItems.Where(q => q.EntityId ==
                                                   orderItemId).FirstOrDefault();

            if (orderItem == null)
                throw new Exception();

            orderItem.UpdateQuanity(quantity);
        }
    }

    class OrderItem : EntityMappedByConvention<Order>
    {
        private int _quantity;
        private DateTime modifiedDate;

        public OrderItem(Order parent, Guid entityId) : base(parent, entityId)
        {
        }

        public void UpdateQuanity(int quantity)
        {
            //VALIDATE

            var e = new OrderItemUpdatedEvent(quantity, DateTime.UtcNow);
            ApplyEvent(e);
        }

        protected void OnOrderItemUpdated(OrderItemUpdatedEvent e)
        {
            _quantity = e.Quantity;
            modifiedDate = e.ModifiedDate;
        }
    }

    internal class OrderItemUpdatedEvent : SourcedEntityEvent
    {
        public int Quantity { get; set; }
        public DateTime ModifiedDate { get; set; }

        public OrderItemUpdatedEvent(int quantity, DateTime modifiedDate)
        {
            Quantity = quantity;
            ModifiedDate = modifiedDate;
        }
    }