Core Data attribute uniqueness - objective-c

Question

Let's suppose that I download data from a remote web service. To be more precise I download a list of news from a web service, each news have a newsID that we can consider a primary key for the service so we won't find two equal newsID.
How can I can be sure that i insert only the data with a newsID that doesn't already exist?
I've draft my solution with this method:
- (BOOL)validateNewsID:(NSNumber **)newsID error:(NSError **)error
{
NSError *countError = nil;
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:[self entity]];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"newsID == %#", *newsID];
[fetchRequest setPredicate:predicate];
NSUInteger resultCount = [SharedManagedObjectContext countForFetchRequest:fetchRequest error:&countError];
if (countError) {
OLog(countError);
*error = countError;
return NO;
}
if (resultCount > 0) {
NSString *errorString = #"NewsID should be unique";
NSDictionary *userInfoDict = [NSDictionary dictionaryWithObject:errorString
forKey:NSLocalizedDescriptionKey];
NSError *uniquenessError = [[[NSError alloc] initWithDomain:kNewsValidationDomain
code:kNewsValidationUniquenessCode
userInfo:userInfoDict] autorelease];
*error = uniquenessError;
return NO;
}
return YES;
}
but doesn't works as expect because, I suppose, when i perform the fetch request i find my same object previously insert in the context. Am I wrong?
How can i fix it?

Answer 1

Seems like you would find the same object you're trying to validate. Try one of these:
Execute the fetch request and remove the current object from the result array before checking for other entries.
Adjust the predicate so it excludes the current object, if possible.
Change resultCount > 0 to resultCount > 1.