public class OrderRecord
    {
        public virtual int Id { get; set; }
        public virtual int CustomerId { get; set; }
        public virtual DateTime CreatedAt { get; set; }
        public virtual decimal SubTotal { get; set; }
        public virtual decimal Vat { get; set; }
        public virtual OrderStatus Status { get; set; }
        public virtual IList<OrderDetailRecord> Details { get; private set; }
        public virtual string PaymentServiceProviderResponse { get; set; }
        public virtual string PaymentReference { get; set; }
        public virtual DateTime? PaidAt { get; set; }
        public virtual DateTime? CompletedAt { get; set; }
        public virtual DateTime? CancelledAt { get; set; }

        public virtual decimal Total
        {
            get { return SubTotal + Vat; }
            private set { }
        }

        public virtual string Number
        {
            get { return (Id + 1000).ToString(CultureInfo.InvariantCulture); }
            private set { }
        }

        public OrderRecord()
        {
            Details = new List<OrderDetailRecord>();
        }

        public virtual void UpdateTotals()
        {
            var subTotal = 0m;
            var vat = 0m;

            foreach (var detail in Details)
            {
                subTotal += detail.SubTotal;
                vat += detail.Vat;
            }

            SubTotal = subTotal;
            Vat = vat;
        }

SchemaBuilder.CreateTable("OrderRecord", t => t
                    .Column<int>("Id", c => c.PrimaryKey().Identity())
                    .Column<int>("CustomerId", c => c.NotNull())
                    .Column<DateTime>("CreatedAt", c => c.NotNull())
                    .Column<decimal>("SubTotal", c => c.NotNull())
                    .Column<decimal>("Vat", c => c.NotNull())
                    .Column<string>("Status", c => c.WithLength(50).NotNull())
                    .Column<string>("PaymentServiceProviderResponse", c => c.WithLength(null))
                    .Column<string>("PaymentReference", c => c.WithLength(50))
                    .Column<DateTime>("PaidAt", c => c.Nullable())
                    .Column<DateTime>("CompletedAt", c => c.Nullable())
                    .Column<DateTime>("CancelledAt", c => c.Nullable())

                    );

public OrderRecord CreateOrder(int customerId, IEnumerable<ShoppingCartItem> items)
{

    if (items == null)
        throw new ArgumentNullException("items");

    // Convert to an array to avoid re-running the enumerable
    var itemsArray = items.ToArray();

    if (!itemsArray.Any())
        throw new ArgumentException("Creating an order with 0 items is not supported", "items");

    var order = new OrderRecord
    {
        CreatedAt = _dateTimeService.Now,
        CustomerId = customerId,
        Status = OrderStatus.New
    };

    _orderRepository.Create(order);