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

Question

I'm using Core Data to locally persist results from a Web Services call. The web service returns the full object model for, let's say, "Cars" - could be about 2000 of them (and I can't make the Web Service return anything less than 1 or ALL cars.

The next time I open my application, I want to refresh the Core Data persisted copy by calling the Web Service for all Cars again, however to prevent duplicates I would need to purge all data in the local cache first.

Is there a quicker way to purge ALL instances of a specific entity in the managed object context (e.g. all entities of type "CAR"), or do I need to query them call, then iterate through the results to delete each, then save?

Ideally I could just say delete all where entity is Blah.

Answer 1

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