Allow only one instance of a given Core Data Entity

Question

What would be the best way to do this, without violating the design principles?

One example would be fetching all previous instances of that entity within awakeFromInsert, and then throwing an exception (or whatever one wants to do) if something is found. But this way is bad, because surely it's not good to access a MOC within a model object.

Answer 1

You can't actually enforce a singleton pattern on a Core Data object graph. You can try to restrict the insertion of another instance but that is cumbersome.

In my experience, the belief that you need a singleton managed object is an indication of a poor design. Singletons are used for control inside a program. Core Data objects are supposed to represent real-world objects, events or conditions and real-world objects, events and conditions don't appear as singletons or anything like them.

You are probably attempting to put controller or even view logic inside the data model. You might want to rethink you data model.

Answer 2

I know this is an old thread, but I actually came across an instance where I wanted to save a single instance of an entity. Here is some example code that might help out others out that come across this.

My solution was to clear an entity (Profile), on saving that same entity to Core Data, if it had >= 1 instance:

- (IBAction)saveProfile:(id)sender {

    NSManagedObjectContext *context = [self managedObjectContext];

    if ([self.profile count] >= 1) {

        NSFetchRequest * allCars = [[NSFetchRequest alloc] init];
        [allCars setEntity:[NSEntityDescription entityForName:@"Profile" inManagedObjectContext:context]];
        [allCars setIncludesPropertyValues:NO];

        NSError * error = nil;
        NSArray * cars = [context executeFetchRequest:allCars error:&error];

        for (NSManagedObject * car in cars) {
            [context deleteObject:car];
        }

        NSError *saveError = nil;
        [context save:&saveError];

    }

    // Create a new managed object
    NSManagedObject *profile = [NSEntityDescription insertNewObjectForEntityForName:@"Profile" inManagedObjectContext:context];

    // Code to save information to persistent data store

}

As expected, in viewDidLoad I pull the entity's (Profile) information and populate the view if profile == 1 instance:

if ([self.profile count] == 1) {
   // Code to pull information from persistent data store and populate the view
}