Fluent NHibernate - 如何将外键列映射为属性

public class Order
{
    public virtual int Id { get; set; }
    public virtual Customer Customer { get; set; }
    public virtual IList<OrderItem> Items { get; set; }
}

public class OrderItem
{
    public virtual int Id { get; set; }
    public virtual Order Order { get; set; }
    public virtual Product Product { get; set; }
    public virtual int Quantity { get; set; }
}

public class Product
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual decimal Price { get; set; }
}

public class OrderMap : ClassMap<Order>
{
    public OrderMap()
    {
        Id(x => x.Id);
        References(x => x.Customer);
        HasMany(x => x.Items);
    }
}

public class OrderItemMap : ClassMap<OrderItem>
{
    public OrderItemMap()
    {
        Id(x => x.Id);
        References(x => x.Order);
        References(x => x.Product);
        Map(x => x.Quantity);
    }
}

public class ProductMap : ClassMap<Product>
{
    public ProductMap()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        Map(x => x.Price);
    }
}

var configuration = new Configuration();
configuration.Configure();
configuration.AddMappingsFromAssembly(typeof(OrderMap).Assembly);