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How do I make a custom validation (for uniqueness) in Core Data?
-(BOOL)validateValue:(__autoreleasing id *)value forKey:(NSString *)key error:(NSError *__autoreleasing *)error {
    [super validateValue:value forKey:key error:error];

    // Validate uniqueness of my_unique_id
    if([key isEqualToString:@"my_unique_id"]) {
        NSFetchRequest * fetch = [[NSFetchRequest alloc] init];
        [fetch setEntity:[NSEntityDescription entityForName:[self.entity name]
               inManagedObjectContext:self.managedObjectContext]];

        NSPredicate *predicate = [NSPredicate
            predicateWithFormat:@"my_unique_id = %@",[self valueForKey:key]];

        fetch.predicate = predicate;

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

        if (count > 1) {
            // Produce error message...

            // Failed validation:
            return NO;
        }


    }

    return YES;
}