title: Core Data fetchedProperty issue on iOS 7
body: <p>it has been few days that I am struggling with core Data. My application works very well on iOS 6, when I use iOS 7 on the Simulator I got this error:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
      reason: '*** -constantValue only defined for abstract class. 
      Define -[NSKeyPathExpression constantValue]!'
  </code></pre>
  
  <p>I use fetchedproperties (n2) in my DataModel and I retriev them as :</p>
  
  <pre><code>UIImage *image2 = [UIImage imageNamed:[NSString stringWithFormat:@"%@.png",
      [(_maqam.n2)[0] valueForKey:@"noteImgPath"]]];
  [B2 setImage:image2 forState:UIControlStateNormal];
  </code></pre>
  
  <p>Thank you in advance for any help.</p>