title: Check if CoreData attribute is empty
body: <p>I am hoping to find a way to check, if a CoreData attribute is empty. The attribute itself is of type binary data. If the attribute is empty then I could tell my class to download and save some data into this attribute.</p>
  
  <p>According to CoreData Documentation, you should not keep fetching to see if objects exists. I am wondering if there is even a way to possibly do this? without breaking this 'law'?</p>
  
  <p>This is my first attempt at using CoreData. I am adding it to my code afterwards, which is slightly more painful, but as a whole so far everything seems to be going okay. I just need to figure out a logical way of checking if attribute has values. If it doesn't then I need to download and save the new data, if it does then I just use what's in the attribute.</p>
  
  <p><strong>Update :</strong></p>
  
  <p>I just found this method in the CoreData framework that I have been reading though trying to catch a break on this. Not sure if it would help.. what do you guys think?</p>
  
  <blockquote>
    <p><a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObject_Class/Reference/NSManagedObject.html#//apple_ref/occ/instm/NSManagedObject/willAccessValueForKey:" rel="noreferrer">willAccessValueForKey</a>: Provides support for key-value observing access
    notification.</p>
    
    <ul>
    <li>(void)willAccessValueForKey:(NSString *)key Parameters key The name of one of the receiver's properties. Discussion See
    didAccessValueForKey: for more details. You can invoke this method
    with the key value of nil to ensure that a fault has been fired, as
    illustrated by the following example.</li>
    </ul>
    
    <p>[aManagedObject willAccessValueForKey:nil];</p>
  </blockquote>
  
  <p>Not sure really.. the things that I dont understand is <strong>Provides support for key-value observing access notification.</strong> ???</p>