NSPredicate filter the data via relationship rather than a property

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].

Now I want pass the Event object and get all days using NSFetchedResultsController and feed all days in the table. Here is my 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  ->  ??
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"event = %@", self.currentEvent];
[fetchRequest setPredicate:predicate];

//set entity
[fetchRequest setEntity开发者_StackOverflow: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];

Somehow, the NSFetchedResultsController instance will always return the same thing for different event. How should I fix the NSPredicate?

I figured it out:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF IN %@", self.currentEvent.days];