title: 'NSInvalidArgumentException', reason: 'keypath Patient.nachname not found in entity <NSSQLEntity Patient id=1>'
body: <p>Why I am getting a NSInvalidArgumentException?</p>
  
  <p>My table view controller code:</p>
  
  <pre><code>- (NSFetchedResultsController *)fetchedResultsController {
  
  CoreDataHelper *cdh = [(MRMedSafeAppDelegate *) [[UIApplication sharedApplication] delegate] cdh];
  
  if (_fetchedResultsController != nil) {
      return _fetchedResultsController;
  }
  
  NSFetchRequest  *fetchRequest = [[NSFetchRequest  alloc] init];
  NSEntityDescription  *entity = [NSEntityDescription
                                  entityForName:@"Patient" inManagedObjectContext:cdh.context];
  [fetchRequest setEntity:entity];
  
  NSSortDescriptor  *sort1 = [[NSSortDescriptor  alloc]
                              initWithKey:@"Patient.nachname" ascending:YES];
  NSSortDescriptor  *sort2 = [[NSSortDescriptor  alloc]
                              initWithKey:@"Patient.vorname" ascending:YES];
  
  [fetchRequest setSortDescriptors:[NSArray  arrayWithObjects:sort1, sort2, nil]];
  
  [fetchRequest setFetchBatchSize:20];
  
  NSFetchedResultsController *theFetchedResultsController =
  [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                      managedObjectContext:cdh.context
                                        sectionNameKeyPath:nil
                                                 cacheName:nil];
  self.fetchedResultsController = theFetchedResultsController;
  _fetchedResultsController.delegate = self;
  
  return _fetchedResultsController;
  </code></pre>
  
  <p>}</p>
  
  <p>My Patient.h:</p>
  
  <pre><code>@interface Patient : NSManagedObject
  @property (nonatomic, retain) NSDate * geburtsdatum;
  @property (nonatomic, retain) NSString * nachname;
  @property (nonatomic, retain) NSString * vorname;
  @property (nonatomic, retain) NSNumber * weiblich;
  @end
  </code></pre>
  
  <p>My Patient.m:</p>
  
  <pre><code>#import "Patient.h"
  @implementation Patient
  @dynamic geburtsdatum;
  @dynamic nachname;
  @dynamic vorname;
  @dynamic weiblich;
  @end
  </code></pre>
  
  <p>Patient.h and Patient.h generated by XCode 5 out of the Model.xcdatamodeld file.</p>
  
  <p>Why I am getting an exception? </p>
  
  <blockquote>
    <p><em>*</em> Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'keypath Patient.nachname not found in entity '</p>
  </blockquote>
  
  <p>Patient is the only class, there are no other model classes and no relationships in the model.</p>