public class Order
    {
        public virtual int ORDER_ID { get; protected set; }
        public virtual string ORDER_NAME { get; set; }
        public virtual int CUSTOMER_ID { get; set; }  // Поле, на которое проецируется внешний ключ БД
        public virtual Customer Customer { get; protected set; }
    }

    public OrderMapper()
    {
        Table("ORDERS");
        Id(x => x.ORDER_ID).Column("ORDER_ID").GeneratedBy.Native("SEQ_ORDER_ID");
        Map(x => x.ORDER_NAME).Column("ORDER_NAME");
        References(x => x.Customer, "CUSTOMER_ID");
    }

order.CUSTOMER_ID = order.Customer.CUSTOMER_ID 

public OrderMapper()
    {
        Table("ORDERS");
        Id(x => x.ORDER_ID).Column("ORDER_ID").GeneratedBy.Native("SEQ_ORDER_ID");
        Map(x => x.ORDER_NAME).Column("ORDER_NAME");
        Map(x => x.CUSTOMER_ID).Column("CUSTOMER_ID");
        References(x => x.Customer, "CUSTOMER_ID");
    }

public OrderMapper()
    {
        Table("ORDERS");
        Id(x => x.ORDER_ID).Column("ORDER_ID").GeneratedBy.Native("SEQ_ORDER_ID");
        Map(x => x.ORDER_NAME).Column("ORDER_NAME");
        Map(x => x.CUSTOMER_ID).Column("CUSTOMER_ID").ReadOnly();
        References(x => x.Customer, "CUSTOMER_ID");
    }

\npublic void SaveNewOrder(Order order)\n{\n...\n  order.Customer = session.Load<Customer>(order.CUSTOMER_ID);\n...\n}\n

Customer customer = new Customer { CustomerID = 3 };

public class Customer
    {
        public virtual int CUSTOMER_ID { get; protected set; }
...
     }