CoreData check if to-many-relationship contains object - core-data

Question

I have the following data model in my app:
Basically I want to store country names aswell as city names. Each city belongs to one country and one country contains 0 - n cities.
Before I now add new cities to a certain country I need to know if the country already contains a city with this name.
Until now I do this:
- (BOOL)countryForName:(NSString *)countryName containsCity:(NSString *)cityName {
Countries *country = [self countryForName:countryName];
NSSet *cityNames = [country valueForKey:#"cities"];
for (Cities *city in cityNames) {
if ([city.cityName isEqualToString:cityName]) {
return YES;
}
}
return NO;
}
This obviously is very slow, what I need is an equivalent fetch with a correct predicate. I perform a search for one entity like this:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Countries" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"countryName" ascending:YES];
NSArray *sortDescriptors = #[sortDescriptor];
fetchRequest.sortDescriptors = 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:self.managedObjectContext sectionNameKeyPath:nil cacheName:#"GetCountryList"];
aFetchedResultsController.delegate = self;
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"countryName == %#", countryName];
[aFetchedResultsController.fetchRequest setPredicate:predicate];
But how do I do this if a search involves multiple entities? Or in other words: How do I add unique city names for one country only?

Answer 1

If you want to check that a given country object already contains a city with a name,
you can do
Countries *country = ...
NSString *cityName = ...
if ([[country valueForKeyPath:#"cities.name"] containsObject:cityName]) {
// ...
}
Or, using a fetch request:
NSString *countryName = ...
NSString *cityName = ...
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:#"Countries"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"countryName == %# AND ANY cities.cityName == %#", countryName, cityName];
[fetchRequest setPredicate:predicate];
NSError *error;
NSUInteger count = [self.managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (count == NSNotFound) {
// error
} else if (count == 0) {
// no matching country
} else {
// at least one matching country
}
Note that a NSFetchedResultsController is normally used to display the contents of a
fetch request in a table view, so it is not needed here.