The given key was not present in the dictionary NHibernate on Delete - nhibernate-mapping

Question

I am having problems deleting an entity from the database using NHibernate v3.
foreach (ShoppingCart s in query )
{
_session.Delete(s);
trans.Commit();
break;
}
"The given key was not present in the dictionary"
There really isn't any more specific information than that. Basically, I'm using two entities as shown below:
| ShoppingCart |==========> | Product |
ShoppingCart PK: ShoppingCartID, ProductID (also is FK)
Product PK: ProductID (no FK)
... A ShoppingCart entity which has many Products inside it.
My ShoppingCart mapping entity look like this:
<class name="ShoppingCart" table="LC_SHOPPINGCART" lazy="true" >
<composite-id>
<key-property name="CartID" column="CARTID" />
<key-property name="ProductID" column="PRODUCTID" />
<key-property name="Size" column="SIZE" />
<key-property name="Color" column="COLOR" />
</composite-id>
<many-to-one foreign-key="ProductID" insert="false" update="false" lazy="false" name="Product">
<column name="PRODUCTID" sql-type="INT" not-null="true" />
</many-to-one>
<property name="Quantity">
<column name="QUANTITY" sql-type="INT" not-null="false" />
</property>
<property name="DateAdded">
<column name="DATEADDED" sql-type="SMALLDATETIME" not-null="false" />
</property>
<property name="Price">
<column name="PRICE" sql-type="MONEY" not-null="false" />
</property>
And the Entity class for ShoppingCart:
public ShoppingCart() { }
[Key]
public virtual string CartID { get; set; }
public virtual int ProductID { get; set; }
public virtual string Size { get; set; }
public virtual string Color { get; set; }
public virtual Product Product { get; set; }
public virtual System.Nullable<int> Quantity { get; set; }
public virtual DateTime DateAdded { get; set; }
public virtual System.Nullable<decimal> Price { get; set; }
My Product Mapping looks like this:
<class name="Product" table="LC_PRODUCT" lazy="true" >
<id name="ProductID" column="ProductID">
<generator class="native"></generator>
</id>
<list name="ShoppingCarts"
cascade="all-delete-orphan">
<key column="ProductID"></key>
<index column="CartID"></index>
<one-to-many class="ShoppingCart"></one-to-many>
</list>
<property name="Name">
<column name="NAME" sql-type="NVARCHAR" not-null="false" />
</property>
<property name="Description">
<column name="DESCRIPTION" sql-type="VARCHAR" not-null="false" />
</property>
<property name="Price">
<column name="PRICE" sql-type="MONEY" not-null="false" />
</property>
<property name="Image1FileName">
<column name="IMAGE1FILENAME" sql-type="NVARCHAR" not-null="false" />
</property>
<property name="Image2FileName">
<column name="IMAGE2FILENAME" sql-type="NVARCHAR" not-null="false" />
</property>
<property name="OnCatalogPromotion">
<column name="ONCATALOGPROMOTION" sql-type="BIT" not-null="false" />
</property>
<property name="OnDepartmentPromotion">
<column name="ONDEPARTMENTPROMOTION" sql-type="BIT" not-null="false" />
</property>
<property name="Sizeable">
<column name="SIZEABLE" sql-type="BIT" not-null="false" />
</property>
<property name="Colorable">
<column name="COLORABLE" sql-type="BIT" not-null="false" />
</property>
<property name="DateAdded">
<column name="DATEADDED" sql-type="SMALLDATETIME" not-null="false" />
</property>
<property name="TotalRemaining">
<column name="TOTALREMAINING" sql-type="INT" not-null="false" />
</property>
</class>
...And the corresponding Product entity class:
public Product() { }
[Key]
public virtual int ProductID { get; set; }
public virtual IList<ShoppingCart> ShoppingCarts { get; set; }
public virtual string Name { get; set; }
public virtual string Description { get; set; }
public virtual System.Nullable<decimal> Price { get; set; }
public virtual string Image1FileName { get; set; }
public virtual string Image2FileName { get; set; }
public virtual bool OnCatalogPromotion { get; set; }
public virtual bool OnDepartmentPromotion { get; set; }
public virtual bool Sizeable { get; set; }
public virtual bool Colorable { get; set; }
public virtual DateTime DateAdded { get; set; }
public virtual System.Nullable<int> TotalRemaining { get; set; }
The stack trace is provided as follows:
Server Error in '/ElectronicsRob' Application.
The given key was not present in the dictionary.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
Source Error:
Line 106: //db.lc_ShoppingCarts.DeleteOnSubmit(query);
Line 107: //db.SubmitChanges();
Line 108: new ShoppingCartRepository().DeleteProductFromCart( System.Convert.ToInt32 (Grid1.SelectedDataKey["ProductID"]), LinqShoppingCartAccess.cartID);
Line 109: PopulateControls();
Line 110: }
Source File: c:\inetpub\wwwroot\ElectronicsRob\LINQControls\ControlTemplates\LinqShoppingCart.ascx.cs
Line: 108
Stack Trace:
[KeyNotFoundException: The given key was not present in the dictionary.]
NHibernate.Engine.StatefulPersistenceContext.RemoveEntity(EntityKey key) in d:\CSharp\NH\nhibernate\src\NHibernate\Engine\StatefulPersistenceContext.cs:435
NHibernate.Action.EntityDeleteAction.Execute() in d:\CSharp\NH\nhibernate\src\NHibernate\Action\EntityDeleteAction.cs:88
NHibernate.Engine.ActionQueue.Execute(IExecutable executable) in d:\CSharp\NH\nhibernate\src\NHibernate\Engine\ActionQueue.cs:136
NHibernate.Engine.ActionQueue.ExecuteActions(IList list) in d:\CSharp\NH\nhibernate\src\NHibernate\Engine\ActionQueue.cs:125
NHibernate.Event.Default.AbstractFlushingEventListener.PerformExecutions(IEventSource session) in d:\CSharp\NH\nhibernate\src\NHibernate\Event\Default\AbstractFlushingEventListener.cs:241
NHibernate.Event.Default.DefaultFlushEventListener.OnFlush(FlushEvent event) in d:\CSharp\NH\nhibernate\src\NHibernate\Event\Default\DefaultFlushEventListener.cs:20
NHibernate.Impl.SessionImpl.Flush() in d:\CSharp\NH\nhibernate\src\NHibernate\Impl\SessionImpl.cs:1477
NHibernate.Transaction.AdoTransaction.Commit() in d:\CSharp\NH\nhibernate\src\NHibernate\Transaction\AdoTransaction.cs:190
ObjexxRepositories.ShoppingCartRepository.DeleteProductFromCart(Nullable`1 prodID, String cartID) in C:\Users\Rob\Documents\Visual Studio 2010\Projects\NGC6744\ObjexxRepositories\Concrete\ShoppingCartRepository.cs:78
LINQControls_LinqShoppingCart.Grid1_RowDeleting(Object sender, GridViewDeleteEventArgs e) in c:\inetpub\wwwroot\ElectronicsRob\LINQControls\ControlTemplates\LinqShoppingCart.ascx.cs:108
System.Web.UI.WebControls.GridView.HandleDelete(GridViewRow row, Int32 rowIndex) +814
System.Web.UI.WebControls.GridView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) +431
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3707
Any help is appreciated, thank you!