Question

What’s the best way to make an NSManagedObject attribute Unique?

For example, should willSave or validateForInsert: or validateForUpdate: managed object methods be used to check if the value entered in attributeUnique is Unique? And, how exactly?

Answer 1

I ended up writing an NSManagedObject category method, and using the NSManagedObject‘s validation method:

- (BOOL)isValueUniqueForKey:(NSString *)key {
    NSEntityDescription *entity = [NSEntityDescription entityForName:[self.entity name]
                                              inManagedObjectContext:self.managedObjectContext];
    id value = [self valueForKey:key];

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@ = %@", key, value];
    fetchRequest.predicate = predicate;

    NSError *error = nil;
    NSUInteger count = [self.managedObjectContext countForFetchRequest:fetchRequest
                                                                 error:&error];

    if (count > 1) {
        return NO;
    }

    return YES;
}