Fetch request with multiple predicates? - iphone

Question

I have an entity Tags, and I want to perform a fetch to get all Tags and assign that to a NSFetchedResultsController. But, I want the first object in the fetched results to be the Tag with tagName property equal to "All", then the rest sorted by alphabetical order. Currently I'm doing this, which just returns all of the tags in alphabetical order, but I want the tag named "All" to be first always, then the rest in alphabetical order.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Tag" inManagedObjectContext:appDelegate.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *lastDescriptor =
[[[NSSortDescriptor alloc] initWithKey:#"tagName"ascending:YES selector:#selector(localizedCaseInsensitiveCompare:)] autorelease];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:lastDescriptor]];

Answer 1

You need to use a comparator created by you, not one default, so you can do something like that:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Tag" inManagedObjectContext:appDelegate.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *lastDescriptor =
[[[NSSortDescriptor alloc] initWithKey:#"tagName" ascending:YES comparator:^NSComparisonResult(NSString* tag1, NSString* tag2) {
if ([tag1 isEqualToString:#"All"]) return NSOrderedAscending;
if ([tag2 isEqualToString:#"All"]) return NSOrderedDescending;
return [tag1 compare:tag2];
}] autorelease];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:lastDescriptor]];
Edit
Like #Andrew Madsen told you can use a comparator, I do not see that he already have answered your question.
You can use a selector too, just need to implement it in your model a method to do the comparison, what I made a mistake and believed that was what you are doing, from apple doc:
selector
The method to use when comparing the properties of objects,
for example caseInsensitiveCompare: or localizedCompare:. The selector
must specify a method implemented by the value of the property
identified by keyPath. The selector used for the comparison is passed
a single parameter, the object to compare against self, and must
return the appropriate NSComparisonResult constant. The selector must
have the same method signature as:
- (NSComparisonResult)localizedCompare:(NSString *)aString

Answer 2

You can do this with NSSortDescriptor itself using a custom compare method. However, you can also sort the results of the fetch request after you get them back. NSArray has a method called -sortedArrayUsingComparator: that allows you to sort the array using a block, making custom sort behavior like this pretty easy. See below for an example:
NSArray *sortedResults = [results sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
NSString *tag1 = [obj1 tagName];
NSString *tag2 = [obj2 tagName];
BOOL tag1IsAll = [tag1 isEqualToString:#"All"];
BOOL tag2IsAll = [tag2 isEqualToString:#"All"];
if (tag1IsAll && !tag2IsAll) return NSOrderedAscending;
if (tag2IsAll && !tag1IsAll) return NSOrderedDescending;
return [tag1 compare:tag2];
}];