How to use groupBy with MagicalRecord? - core-data

Question

I am stuck on how to use groupBy with MagicalRecord.
I have a list of countries with venues
Country -<< Venues
I need to group all the venues by the country and sort the countries by name.
But I am not sure how to do this with MagicalRecord.
I have tried to use a NSFetchedController but sometimes it crashes saying that the array is nil or 0 length.
Other times, it only ever sees 1 category when there are multiple.
Finally, I am not sure how to execute the fetch on an entity.
ie;
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
_objects = [NSMutableArray array];
self.fetchedResultsController = [self fetchedResultsController];
[Venue performFetch:self.fetchedResultsController];
// At this point how do I make the Venue findAllSortedBy work on the performFetch?
_objects = [NSMutableArray arrayWithArray:[Venue findAllSortedBy:#"name" ascending:YES inContext:[NSManagedObjectContext defaultContext]]];
self.title = #"Venues";
}
- (NSFetchedResultsController *)fetchedResultsController {
if (!fetchedResultsController) {
fetchedResultsController = [Venue fetchAllSortedBy:#"name"
ascending:YES
withPredicate:nil
groupBy:#"country"
delegate:self];
}
return fetchedResultsController;
}
-(NSString*)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
NSLog(#"Section = %d", section);
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
NSLog(#"sectionInfo = %#", sectionInfo);
return #"Header";
}
- (UITableViewCell *)tableView:(UITableView *)tv cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tv dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier];
}
[self configureCell:cell atIndexPath:indexPath];
return cell;
}
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath {
Venue *v = [_objects objectAtIndex:indexPath.row];
cell.textLabel.text = v.name;
}
I am not sure if I am doing this right.
The above will put everything in 1 country (when there are multiple) and the log will report;
CoreData: error: (NSFetchedResultsController) A section returned nil value for section
name key path 'country'. Objects will be placed in unnamed section
It seems not to see the different countries and I do not think I've done the GroupBy command correctly.
Thus, how do I do a GroupBy command with MagicalRecord?
Many thanks.

Answer 1

self.fetchedVenues = [Venue MR_fetchAllGroupedBy:#"country" withPredicate:nil sortedBy:#"name" ascending:YES];

Answer 2

This error is telling you that of all your Venue objects, there is at least one in your result set that does not have a value for "country". You need to verify that you are indeed filling in this field and saving it properly prior to fetching.
And FYI, in your viewDidLoad method, you don't need all that code. Simply do something like:
- (void) viewDidLoad;
{
[super viewDidLoad];
self.fetchedVenues = [Venue fetchAllSortedBy:#"name" ascending:YES withPredicate:nil groupBy:#"country" delegate:self];
}
fetchAllSortedBy... will perform the fetch for you, and log errors, etc. That is the point of a helper framework like MagicalRecord.