Code for uniderectional association

gistfile1.cs

public class Order
{
    public virtual Guid Id { get; set; }
    public virtual int Version { get; set; }
    public virtual DateTime OrderTime { get; set; }
    public virtual ICollection<OrderLine> OrderLines { get; set; }
}

public class OrderLine
{
    public virtual Guid Id { get; set; }
    public virtual string ProductName { get; set; }
    public virtual decimal Price { get; set; }
    public virtual int Quantity { get; set; }
}
...
using (var session = sessionFactory.OpenSession())
using (var tx = session.BeginTransaction())
{
    var order = session.Get<Order>(id);
    order.OrderLines.Add(new OrderLine
    {
        ProductName = "Product 1",
        Price = 9.99M,
        Quantity = 15
    });

    tx.Commit();
}