title: extract property of array into new array
body: <p>i try to extract only the properties into an array to create a NSPredicate for this.
  is there a shorter way for this than creating a mutable array and looping through all objects?</p>
  
  <pre><code>NSMutableArray *extractedIDs = [NSMutableArray array];
  for (SomeObject *aObject in self.addedSearchResults) {
      [addedIDs addObject:aObject.someProperty];
  }
  
  [NSPredicate predicateWithFormat:@"NOT (someOtherProperty IN %@)", addedIDs]
  </code></pre>
  
  <p>can't find something with google (don't really know how to search this problem)</p>