Can NSFetchedResultsController work with NSDictionaryResultType?

Question

I use Core Data to store shops as Entity Shop and on every shop I have an Attribute city, I want to group the shops by city and present a UITableView with all the cities. I use NSFetchedResultsController to fetch the data and refresh the UITableView and since I want to group the cities I set the resultType of the request as NSDictionaryResultType. However now when I use objectAtIndexPath at my NSFetchedResultsController I am getting NSKnownkeysdictionary1 My question is can I make the NSFetchedResultsController handle the NSDictionaryResultTypeor I should drop the use of NSFetchedResultsController in this case and take some other approach?

Answer 1

You have to set the NSExpressionDescription for it to fetch the appropriate value. You could do it like this,

- (NSFetchedResultsController *)fetchedResultsController
{
    if (!_fetchedResultsController) {
        NSExpression *cityKeypath = [NSExpression expressionForKeyPath:@"identifier"];
        NSExpressionDescription *cityDescription = [[NSExpressionDescription alloc] init];
        cityDescription.expression = cityKeypath;
        cityDescription.name = @"City";
        cityDescription.expressionResultType = NSStringAttributeType;


        NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Shop"];
        [fetchRequest setPropertiesToFetch:@[cityDescription]];
        [fetchRequest setPropertiesToGroupBy:@[@"city"]];
        [fetchRequest setResultType:NSDictionaryResultType];

        fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"city" ascending:YES]];
        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                        managedObjectContext:self.managedObjectContext
                                                                          sectionNameKeyPath:nil
                                                                                   cacheName:nil];

       _fetchedResultsController.delegate = self;
        NSError *error;

        [_fetchedResultsController performFetch:&error];
    }
    return _fetchedResultsController;
}

So, you need to provide NSExpressionDescription for the properties you want to fetch. The NSFetchedResultsController result will in in dictionary type like this,

 {
   @"city": "Kansas"
  }
 ...