How to implement a transient property on core data? - ios

Question

I have this core data entity called Countries. This entity has a field called nameOfCountry that contains country names in english. I need to localize this to other languages, so I have created a transient property called nameOfCountryLocalized.
On the Countries class I am importing this category
Countries+NameOfCountryLocalized.h
#import "Countries.h"
#interface Countries (NameOfCountryLocalized)
#property (nonatomic, strong) NSString * nameOfCountryLocalized;
#end
Countries+NameOfCountryLocalized.m
#import "Countries+NameOfCountryLocalized.h"
#import "Countries.h"
#implementation Countries (NameOfCountryLocalized)
#dynamic nameOfCountryLocalized;
-(NSString *) nameOfCountryLocalized {
[self willAccessValueForKey:#"nameOfCountryLocalized"];
NSString *nameLocalized = NSLocalizedString(self.nomePais, nil);
[self didAccessValueForKey:#"nameOfCountryLocalized"];
return nomeLocalizado;
}
-(void)setNameOfCountryLocalized:(NSString *) nameLocalized {
[self willChangeValueForKey:#"nameOfCountryLocalized"];
[self setNomePaisLocalizado:];
[self didChangeValueForKey:#"nameOfCountryLocalized"];
}
#end
when I try to access nameOfCountryLocalized using this from a tableViewController
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Countries" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:#"nameOfCountryLocalized" ascending:YES];
[fetchRequest setSortDescriptors:#[sort]];
[fetchRequest setFetchBatchSize:20];
NSFetchedResultsController *theFetchedResultsController =
[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:self.managedObjectContext
sectionNameKeyPath:nil
cacheName:#"Root"];
_fetchedResultsController = theFetchedResultsController;
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}
I see this error:keypath nameOfCountryLocalized not found in entity
any clues?

Answer 1

The NSFetchedResultsController cannot sort by a transient property. The FRC applies the sort to the underlying SQL store. With a transient property the FRC can group results of a fetch into sections. This group is created by assigned the transient property to sectionNameKeyPath.
Pulling all entities into an array will become a pain once your data grows. The FRC does provide nice support for larger data sets and you don't really want to loose that.
You might be ok assuming that your users don't switch language very often. If that is the case I suggest you write the localized country name into the store as a "normal" property and the FRC can do the sort on that property.
Although when a user does switch language then you would need to update all Country entities.

Answer 2

Your Countries class is not an entity. It is a class for an individual object in the database.
You really should rename it to Country, as it's incorrect to refer to NSManagedObject subclasses as plural. There's a reason Apple didn't name the class NSManagedObjects.
Because your property is added to individual objects but not the entities themselves, it is only available after objects have been fetched from the database. It cannot be used as part of fetching the objects.
You're going to need to fetch the results first (all of them) into an NSArray and then sort the objects by applying the NSSortDescriptor to the NSArray. You might want to do this by creating a wrapper class around NSFetchedResultsController.
Alternatively, put a nameOfCountryLocalized property in the actual database, with english values, and then if the user doesn't use english write to the database changing everything to the correct values. This would allow you to use NSFetchedResultsController exactly as you're trying to do now. I recommend this approach if your database is huge... but it's not, there are only a couple hundred countries in the world so performance is a total non-issue.