Objective-c – fetching objects from core data not in a set

core-dataiphonenspredicateobjective c

I'm trying to fetch objects from core data that are not in a given set, but I haven't been able to get it to work.

For instance, suppose that we have a core data entity named User, which has a few attributes such as userName, familyName, givenName, and active. Given an array of strings representing a set of usernames, we can easily fetch all the users corresponding to that list of usernames:

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"User"
                                          inManagedObjectContext:moc];
[request setEntity:entity];

NSArray *userNames = [NSArray arrayWithObjects:@"user1", @"user2", @"user3", nil];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"userName IN %@", userNames];
[request setPredicate:predicate];
NSArray *users = [moc executeFetchRequest:request error:nil];

However, I want to fetch the complement of that set, i.e., I want all the users in core data that don't have the usernames specified in the userNames array. Does anyone have an idea how to approach this issue? I thought it would be simple enough to add a "NOT" in the predicate (i.e., "userName NOT IN %@"), but Xcode throws an exception saying the predicate format could not be parsed. I also tried using the predicate builder available for fetch requests with no luck. The documentation wasn't particularly helpful either. Suggestions? Comments? Thanks for all your help 🙂

Best Answer

In order to find the objects that aren't in your array, all you have to do is something like this:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"NOT (userName IN %@)", userNames];

That should return a request of all the objects without the ones you specified

iOS 9 and later:

iOS 9 added a new class called NSBatchDeleteRequest that allows you to easily delete objects matching a predicate without having to load them all in to memory. Here's how you'd use it:

Swift 5

let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

do {
    try myPersistentStoreCoordinator.execute(deleteRequest, with: myContext)
} catch let error as NSError {
    // TODO: handle the error
}

Objective-C

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];

NSError *deleteError = nil;
[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];

More information about batch deletions can be found in the "What's New in Core Data" session from WWDC 2015 (starting at ~14:10).

iOS 8 and earlier:

Fetch 'em all and delete 'em all:

NSFetchRequest *allCars = [[NSFetchRequest alloc] init];
[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];
[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectID

NSError *error = nil;
NSArray *cars = [myContext executeFetchRequest:allCars error:&error];
[allCars release];
//error handling goes here
for (NSManagedObject *car in cars) {
  [myContext deleteObject:car];
}
NSError *saveError = nil;
[myContext save:&saveError];
//more error handling here

Iphone – Simple Core Data fetch is very slow

Since you’re not limiting the result set in any way fetching 400,000 objects at once will definitely be a burden on Core Data. There are several ways you can improve performance:

Changing the fetch request's fetchBatchSize limits the number of objects the fetch will keep in memory at a time. This feature is completely transparent to your application, so it's definitely worth a try.

If you don't need fully-fledged objects, you may consider changing the fetch request's resultType to a more appropriate value. Especially if you’re only interested in some of the values of an object, using NSDictionaryResultType is a good idea.

Lastly the fetchLimit and fetchOffset properties allow you to limit the result range, if you want to manage batch processing yourself. This is a good idea if your handling of each of the result objects uses lots of memory because you can wrap each batch in an NSAutoreleasePool (just don’t be tempted to create an autorelease pool for every single result object).

I guess 1 sec. might just be as fast as it gets in your case – even if you resort to a plain Sqlite database. The only further optimisation I can think of is to use one table per language (instead of putting words from all languages into a single table). This, of course, will only work with Sqlite unless you define separate entities for all languages, i. e. take your Words entity as is and make it abstract. Then add subentities like EnglishWord etc. Objects from different entities are stored in separate tables. So, combined with the fetchBatchSize and predicate parameters this should perform similarly to the Sqlite approach with separate tables for all languages.

Best Answer

Related Solutions

Ios – Core Data: Quickest way to delete all instances of an entity

iOS 9 and later:

Swift 5

Objective-C

iOS 8 and earlier:

Iphone – Simple Core Data fetch is very slow

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