How to Determine if a Table Contains Any Records with Core Data?
I haven't seen any other questions quite like this on here, but I'm hoping someone has some insight. I'm just starting to learn Core Data.
Basically, I have two methods and I want to choose which one to call with an if/else statement based on whether or not the "Contacts" table contains any records. Is there a way using core data to check if there are any records in a table?
开发者_如何学CThe best way I've found so far is to set the fetchLimit to 1 and then check to see if anything returns.
[request setFetchLimit:1];
But I keep thinking there has to be a better/easier way. Anyone know or have a good reference I can look at?
Thanks a ton!
Yes, definitely there is a better method. Setup a fetch request as usual, but, instead of actually executing it, simply ask for the number of objects it would have returned if it had been passed to executeFetchRequest:error:
This can be done using
- (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error;
Something like this:
- (int) numberOfContacts{
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSManagedObjectContext *managedObjectContext = yourManagedObjectContext;
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Contacts" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
NSError *error = nil;
NSUInteger count = [managedObjectContext countForFetchRequest:request error:&error];
[request release];
if (!error){
return count;
}
else
return -1;
}
It's not necessarily any better or easier, but you can look for a specific record and then create it if it doesn't exist like this:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Contact"
inManagedObjectContext:[self managedObjectContext]];
[fetchRequest setEntity:entity];
NSError *error;
// Filter based on a predicate
[fetchRequest setPredicate:
[NSPredicate predicateWithFormat:@"identifier == %@", @"1"]];
NSManagedObject *contact = [[managedObjectContext
executeFetchRequest:fetchRequest error:&error] lastObject];
// If the contact was not found
if (!contact)
{
// Create the contact
contact = [NSEntityDescription insertNewObjectForEntityForName:@"Contact"
inManagedObjectContext:managedObjectContext];
[contact setValue:[NSNumber numberWithInt:1] forKey:@"identifier"];
[managedObjectContext save:nil];
}
Marcus Zarra posted some code that demonstrates this in a feed reader app. Marcus is the Core Data master.
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