NHibernate: Using Fluent Nhibernate to save child objects

In my system, I have two entities – ShoppingCart and ShoppingCartItem. Fairly generic use-case. However, when I save my ShoppingCart, none of the items are being saved to the DB.

Within my object, I create a new ShoppingCart object.

ShoppingCart cart = CreateOrGetCart();

I then add an existing Product which I got from the database to the start.

cart.AddItem(product);

This is just a simple wrapper to add the item to the IList.

    public virtual void AddItem(Product product)
    {
        Items.Add(new ShoppingCartItem { Quantity = 1, Product = product });
    }

I then call SaveOrUpdate on the Repository

Repository.SaveOrUpdate(cart);

Which looks like this:

   public T SaveOrUpdate(T entity)
    {
        Session.SaveOrUpdate(entity);
        return entity;
    }

I'm using Fluent NHibernate for the mapping:

    public ShoppingCartItemMap()
    {
        WithTable("ShoppingCartItems");

        Id(x => x.ID, "ShoppingCartItemId");
        Map(x => x.Quantity);

        References(x => x.Cart, "ShoppingCartId").Cascade.SaveUpdate();
        References(x => x.Product, "ProductId");
    }


    public ShoppingCartMap()
    {
        WithTable("ShoppingCarts");

        Id(x => x.ID, "ShoppingCartId");
        Map(x => x.Created);
        Map(x => x.Username);

        HasMany<ShoppingCartItem>(x => x.Items)
            .IsInverse().Cascade.SaveUpdate()
            .WithKeyColumn("ShoppingCartId")
            .AsBag();
    }

Database Schema (SQL Server 2005) is also fairly generic:

CREATE TABLE [dbo].[ShoppingCarts]
(
[ShoppingCartID] [int] NOT NULL IDENTITY(1, 1),
[Username] [nvarchar] (50) NOT NULL,
[Created] [datetime] NOT NULL
)
GO
ALTER TABLE [dbo].[ShoppingCarts] ADD CONSTRAINT [PK_ShoppingCarts] PRIMARY KEY CLUSTERED ([ShoppingCartID])
GO



CREATE TABLE [dbo].[ShoppingCartItems]
(
[ShoppingCartItemId] [int] NOT NULL IDENTITY(1, 1),
[ShoppingCartId] [int] NOT NULL,
[ProductId] [int] NOT NULL,
[Quantity] [int] NOT NULL
)
GO
ALTER TABLE [dbo].[ShoppingCartItems] ADD CONSTRAINT [PK_ShoppingCartItems] PRIMARY KEY CLUSTERED ([ShoppingCartItemId])
GO
ALTER TABLE [dbo].[ShoppingCartItems] ADD CONSTRAINT [FK_ShoppingCartItems_Products] FOREIGN KEY ([ProductId]) REFERENCES [dbo].[Products] ([ProductId])
GO
ALTER TABLE [dbo].[ShoppingCartItems] ADD CONSTRAINT [FK_ShoppingCartItems_ShoppingCarts] FOREIGN KEY ([ShoppingCartId]) REFERENCES [dbo].[ShoppingCarts] ([ShoppingCartID])
GO

When I SaveOrUpdate my ShoppingCart, why isn't any ShoppingCartItems also being saved?

Please help.

Thanks

Ben

UPDATE:
Wrapping it in a transaction providng me with some more info:

Cannot insert the value NULL into column 'ShoppingCartId', table
'WroxPizza.dbo.ShoppingCartItems'; column does not allow nulls.
INSERT fails. The statement has been terminated.

This is because it's a new cart.