fetch request for entity.attribute == #"somevalue" - core-data

Question

How do I setup a fetch request to only pull the data from an entity's attribute with one particular value? This is the basic code I've used before.
-(void)fetchResults
{
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:self.entityDescription.name];
NSString *cacheName = [self.entityDescription.name stringByAppendingString:#"Cache"];
// predicate code
if (self.predicate != nil) {
[fetchRequest setPredicate:self.predicate];
}
// end of predicate code
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:cacheName];
BOOL success;
NSError *error;
success = [self.fetchedResultsController performFetch:&error];
if (!success)
{
NSLog(#"%#", [error localizedDescription]);
}
}
I've been looking at this page: http://bit.ly/KevYwR is this the right direction?
Do I need to use NSPredicate or can I do without?
Thanks for any help, point in the right direction, etc.

Answer 1

Setting up a NSFetchRequest is equivalent to a SELECT statetement in SQL language.
Here a simple example:
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:#"EntityName" inManagedObjectContext:moc]];
NSError *error = nil;
NSArray *results = [moc executeFetchRequest:request error:&error];
// error handling code
The array results contains all the managed objects contained within the sqlite file. If you want to grab a specific object (or more objects) you need to use a predicate with that request. For example:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"attribute == %#", #"Some Value"];
[request setPredicate:predicate];
In this case results contains the objects where attribute is equal to Some Value. Setting a predicate is equal to put the WHERE clause in a SQL statement.
Note
I suppose that the name of the entity is EntityName and its property is called attribute which is of type string.
For further info I suggest you to read the Core Data programming guide and NSFecthRequest class reference.
http://developer.apple.com/library/iOS/#documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html
Hope it helps.