Rails - May 2006 - problem with validates_presence_of :on=> :update

I''m having what''s probably a stupid-newbie problem with
validates_presence_of and I''m hoping someone will point me in the right
direction.

Bottom line is that validates_presence_of does not seem to be
''firing'' when I use the :on => :update option.  If I
don''t include the option, the validation does fire.

The problem is that I want to save the record before I present it to the user to
update.  When it comes back from the user I want to make sure they''ve
updated a couple of fields and so that''s where I need to do the
validation.

When I leave off the option, the validation is firing on the initial save. 
That''s ''bad'' because the data is not _supposed_ to be
there at that point.  When I include the :on => :update, it doesn''t
fire on the initial save.  That''s ''good.''  But if I
leave the fields that I''m validating-on empty when I submit the form,
the @actor.update happens without throwing an error.  The record is definitely
getting updated in the database.  That''s very ''bad.''

Anybody have any ideas?
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