Why isn't "DbContext.SaveChanges()" throwing "DbUpdateConcurrencyException"
===

I'm experimenting with POCO using "DbContext" and don't understand how to trigger an optimistic concurrency exception. When I call "DbContext.SaveChanges()" at a time when it should fail concurrency, it returns zero instead of throwing "DbUpdateConcurrencyException" or "OptimisticConcurrencyException" for that matter. It's unclear which should be thrown (presumably the former) but I've read this in detail http://blogs.msdn.com/b/adonet/archive/2011/02/03/using-dbcontext-in-ef-feature-ctp5-part-9-optimistic-concurrency-patterns.aspx as well as many articles and can't find anything that addresses the situation. Is there something I need to explicitly do to generate the exception. Note that my POCO classes and "DBContext" derivative were automatically generated by the "EF 4.x DBContext Generator" and the field being updated has its "Concurrency Mode" set to "Fixed" (though I don't think this has any impact in the POCO environment).