title: Setting property in CoreData object results in save error
body: <p>When I update a NSManagedObject and save it, I get an error: The operation couldn’t be completed. (Cocoa error 1560.)</p>
  
  <ul>
  <li>the object is already in the database, so in fact I am doing an update when saving it</li>
  <li>when I save the object prior to modifying it, no error</li>
  <li>I modify it by setting a property of the object. Using KVP has the same result</li>
  <li>the original save (the object has been created shortly before) and saving after the update are done all in the main thread. The object is never touched from another thread.</li>
  </ul>
  
  <p>The method validateForUpdate: always returns NO. I am not sure if this method should return YES even if the object has not been changed: if object X is saved without error and then I call validateForUpdate: it returns NO.</p>
  
  <p>Any ideas?</p>