Objective-c – Fastest way to check if an object exists in Core Data or not

core-dataiphoneobjective c

I want to see if an object is persisted in Core Data or not. For example, I have Friends in Core Data, and I identify them by firstName. I can query core data to see if "George" is known. If the result set array contains more than zero objects, I know George is there. But Core Data loads the whole thing into memory, and I actually just want to know if George is stored or not.

How would I do it the most efficient way?

Best Answer

Setup a Core Data request and, instead of actually issuing the query, do the following:

NSError *error = nil;
NSUInteger count = [managedObjectContext countForFetchRequest:request
                                                        error:&error];
if (!error) {
    return count;
} else {
  return 0;
}

In practice, the method countForFetchRequest:error: returns the number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:.

Edit: ^{(by Regexident)}

As Josh Caswell correctly commented, the correct way to handle errors is either this:

if (count == NSNotFound) {
    NSLog(@"Error: %@", error);
    return 0;
}
return count;

or this (without error logging):

return (count != NSNotFound) ? count : 0;

Related Solutions

Objective-c – Cocoa Core Data efficient way to count entities

I don't know whether using NSFetchedResultsController is the most efficient way to accomplish your goal (but it may be). The explicit code to get the count of entity instances is below:

// assuming NSManagedObjectContext *moc

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:moc]];

[request setIncludesSubentities:NO]; //Omit subentities. Default is YES (i.e. include subentities)

NSError *err;
NSUInteger count = [moc countForFetchRequest:request error:&err];
if(count == NSNotFound) {
  //Handle error
}

[request release];

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

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