Need to modify Master datagrid to show only 1 record based on combobox selection (VS2012 framework 4)

(WPF newbie) I'm trying out a walkthrough for creating a WPF related data sample -- at:

http://msdn.microsoft.com/en-us/library/dd465158.aspx

I would like to modify the following so that the master grid only displays 1 records based on the combobox selection (detail grid stays the same).  In the picture below, when I select a record from each row from the combobox -- 2 things:

1) the combobox only lists 1 orderID per row

2) when I select another orderID the previous selection (orderID) remains visible.

Did I miss a step maybe?  (like maybe it was assumed that I should know how to do something else here) or does the xaml need to be specifically modified to get the desired functionality?  I want the master grid to show only 1 record at a time based on my selection from the OrderID combobox (1st column).  How to do this?  Purpose: How do I know which OrderID the detail data is based on here?

Thanks

Rich P