title: Get all results of a NSFetchRequest in an NSArray
body: <p>In my app I do this thing to abtain all specific value from an entity</p>
  
  <pre><code>NSManagedObjectContext *context = [[self sharedAppDelegate] managedObjectContext];
  NSError *error = nil;
  NSFetchRequest *req = [NSFetchRequest fetchRequestWithEntityName:@"Structure"];
  [req setPropertiesToFetch:@[@"id_str"]];
  [req setResultType:NSDictionaryResultType];
  
  NSArray *id_str_BD = [context executeFetchRequest:req error:&amp;error];
  </code></pre>
  
  <p>int this way I obtain a <code>NSArray</code> of <code>NSDictionary</code>ies but I want directly an array of values.
  What's a fast way to obtain it without loops?</p>