I have a 2 database tables defined as in erd.jpg. In the mapper (see jpg's) each table has only one field flagged with the key icon. I am confused.

When you configure your tables with a guid primary key, are you using a Char(32) data type and putting the guid in that, or are you using the UNIQUEIDENTIFIER data type and storing the guids there? Because if you're using the Char(32) type, you might consider changing them to the UNIQUEIDENTIFIER because it's half the size and optimized for guids. Also your field properties on the business objects would return a System.Guid rather than a string. Just a thought.