title: NSPredicate filter the data via relationship rather than a property
body: <p>Here is my question. I have this model one [Event] has multiple [Day] and there is a relationship called days in [Event] and there is a reverse relationship event in [Day].</p>
  
  <p>Now I want pass the Event object and get all days using NSFetchedResultsController and feed all days in the table. Here is my code:</p>
  
  <pre><code>// Create the fetch request for the entity.
  NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
  // Edit the entity name as appropriate.
  NSEntityDescription *entity = [NSEntityDescription entityForName:@"Day" inManagedObjectContext:managedObjectContext];
  
  //set predicate  -&gt;  ??
  NSPredicate *predicate = [NSPredicate predicateWithFormat:@"event = %@", self.currentEvent];
  [fetchRequest setPredicate:predicate];
  
  //set entity
  [fetchRequest setEntity:entity];
  
  // Set the batch size to a suitable number.
  [fetchRequest setFetchBatchSize:20];
  
  // Edit the sort key as appropriate.
  NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"date" ascending:YES];
  NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
  
  [fetchRequest setSortDescriptors:sortDescriptors];
  
  // Edit the section name key path and cache name if appropriate.
  // nil for section name key path means "no sections".
  NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
                                                                                              managedObjectContext:managedObjectContext 
                                                                                                sectionNameKeyPath:nil
                                                                                                         cacheName:nil];
  </code></pre>
  
  <p>Somehow, the NSFetchedResultsController instance will always return the same thing for different event.
  How should I fix the NSPredicate?</p>