title: Rails save method if no changes
body: <p>I seem to remember reading somewhere that rails won't commit to the database if no attributes have been changed (presumably as part of active record dirty) is this the case?  I can't seem to find it and am none the wiser having had a quick look through the source code.</p>
  
  <p>If this is false I need to use a before_save callback because I want to run some code based on whether or not a change has occured.</p>
  
  <p>I assume after_save with dirty data won't work??</p>