NSFetchedResultsController and NSOrderedSet relationships

Question

I am having an issue (understanding issue to be honest) with NSFetchedResultsController and the new NSOrderedSet relationships available in iOS 5.

I have the following data-model (ok, my real one is not drawer's and sock's!) but this serves as a simple example:

Drawer and Sock are both NSManagedObjects in a Core Data model/store. On Drawer the socks relationship is a ordered to-many relationship to Sock. The idea being that the socks are in the drawer in a specific order. On Sock the drawer relationship is the inverse of the socks relationship.

In a UIViewController I am drawing a UITableView based on these entities. I am feeding the table using a NSFetchedResultsController.

- (NSFetchedResultsController *)fetchedResultsController1 {
    if (_fetchedResultsController1 != nil) {
        return _fetchedResultsController1;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sock" inManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"drawer.socks" ascending:YES];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    self.fetchedResultsController1 = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[NSManagedObjectContext MR_defaultContext] sectionNameKeyPath:nil cacheName:@"SocksCache"];
    self.fetchedResultsController1.delegate = self;

    return _fetchedResultsController1;    
}

When I run this, I get the following errror: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'

This makes sense to me, as the relationship is a NSOrderedSet and not a single entity to compare against for sorting purposes.

What I want to achieve is for the Socks to appear in the UITableView in the order specified in the socks relationship. I don't really want to have a sort order but NSFetchedResultsController, which is a great component is insisting there has to be one. How can I tell it to use the socks order on the Drawer entity. I don't want the table to show Drawer entities at all.

Note: I am using this within an iOS5 application only, so ordered relationships are available.

Anyone that can offer me any direction, would be greatly appreciated. Thanks for your time.

Edit: So the tableview that display's socks does so for just one drawer. I just want the table view to honor the order that the socks relationship contains. I'm not sure what to set the sort criteria to make sure that happens.

Answer 1

Damien,

You should just make your NSFetchRequest use the array form of the ordered set. It will operate fine. Your controller needs an attribute to sort on. Hence, you'll need to specify that too.

Andrew

Answer 2

I found this thread while looking for an answer to the exact question posed by the OP.

I never found any examples of presenting such data in a tableView without adding the additional sort field. Of course, adding the sort field pretty much eliminates any benefit in using the ordered relationship. So given that I got this working, I thought it might be helpful to others with the same question if I posted my code here. It turned out to be quite simple (much simpler than using an additional sort field) and with apparently good performance. What some folks may not have realized (that includes me, initially) is that the type (NSOrderedSet) of the "ordered, to-many" relationship attribute has a method for getting objectAtIndex, and that NSMUtableOrderedSet has methods for inserting, and removing objectAtIndex.

I avoided using the NSFetchedResultsController, as some of the posters suggested. I've used no array, no additional attribute for sorting, and no predicate. My code deals with a tableView wherein there is one itinerary entity and many place entities, with itinerary.places being the "ordered, to-many" relationship field. I've enabled editing/reordering, but no cell deletions. The method moveRowAtIndexPath shows how I've updated the database with the reordering, although for good encapsulation, I should probably move the database reordering to my category file for the place managed object. Here's the entire TableViewController.m:

//
//  ItineraryTVC.m
//  Vacations
//
//  Created by Peter Polash on 8/31/12.
//  Copyright (c) 2012 Peter Polash. All rights reserved.
//

#import "ItineraryTVC.h"
#import "AppDelegate.h"
#import "Place+PlaceCat.h"
#import "PhotosInVacationPlaceTVC.h"


@interface ItineraryTVC ()

@end

@implementation ItineraryTVC

#define DBG_ITIN YES

@synthesize itinerary ;

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.navigationItem.rightBarButtonItem = self.editButtonItem ;
}

- (void) viewWillAppear:(BOOL)animated
{

    [super viewWillAppear:animated] ;

    UIManagedDocument *doc = UIAppDelegate.vacationDoc;

    [doc.managedObjectContext performBlock:^
     {   // do this in the context's thread (should be the same as the main thread, but this made it work)

         // get the single itinerary for this document

         self.itinerary = [Itinerary setupItinerary: doc ] ;
         [self.tableView reloadData] ;
     }];

}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown );
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView
 numberOfRowsInSection:(NSInteger)section
{
    return [self.itinerary.places count ];
}


- (UITableViewCell *) tableView: (UITableView *) tableView
          cellForRowAtIndexPath: (NSIndexPath *) indexPath
{
    static NSString *CellIdentifier = @"Itinerary Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier: CellIdentifier ];


    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle: UITableViewCellStyleDefault   reuseIdentifier: CellIdentifier];
    }

    Place *place = [self.itinerary.places objectAtIndex:indexPath.row];

    cell.textLabel.text       = place.name;
    cell.detailTextLabel.text = [NSString stringWithFormat:@"%d photos", [place.photos count]];

    return cell;
}


#pragma mark - Table view delegate


- (BOOL)    tableView: (UITableView *) tableView
canMoveRowAtIndexPath:( NSIndexPath *) indexPath
{
    return YES;
}

-(BOOL)     tableView: (UITableView *) tableView
canEditRowAtIndexPath: (NSIndexPath *) indexPath
{
    return YES ;
}

-(void)  tableView: (UITableView *) tableView
moveRowAtIndexPath: (NSIndexPath *) sourceIndexPath
       toIndexPath: (NSIndexPath *) destinationIndexPath
{
    UIManagedDocument * doc = UIAppDelegate.vacationDoc ;

    [doc.managedObjectContext performBlock:^
    { // perform in the context's thread 

        // itinerary.places is the "ordered, to-many" relationship attribitute pointing to all places in itinerary
        NSMutableOrderedSet * places = [ self.itinerary.places  mutableCopy ] ;
        Place *place                 = [ places objectAtIndex:  sourceIndexPath.row] ;

        [places removeObjectAtIndex: sourceIndexPath.row ] ;
        [places insertObject: place   atIndex: destinationIndexPath.row ] ;

        self.itinerary.places = places ;

        [doc saveToURL: doc.fileURL   forSaveOperation: UIDocumentSaveForOverwriting completionHandler: ^(BOOL success) {
            if ( !success ) NSLog(@"Error saving file after reorder, startPos=%d, endPos=%d", sourceIndexPath.row, destinationIndexPath.row) ;
        }];
    }];

}

- (UITableViewCellEditingStyle) tableView: (UITableView *) tableView
            editingStyleForRowAtIndexPath: (NSIndexPath *) indexPath
{
    return ( UITableViewCellEditingStyleNone ) ;
}

- (void) prepareForSegue:(UIStoryboardSegue *) segue   sender: (id) sender
{
    NSIndexPath *indexPath = [self.tableView    indexPathForCell: sender] ;
    PhotosInVacationPlaceTVC  * photosInVacationPlaceTVC = segue.destinationViewController ;

    Place *place = [self.itinerary.places objectAtIndex:indexPath.row ];

    photosInVacationPlaceTVC.vacationPlace        = place ;
    photosInVacationPlaceTVC.navigationItem.title = place.name ;

    UIBarButtonItem *backButton =
    [[UIBarButtonItem alloc] initWithTitle:@"Back" style:UIBarButtonItemStylePlain target:nil action:nil];
    self.navigationItem.backBarButtonItem = backButton;

}


@end

Answer 3

You can give Sock an index attribute and order the socks by them:

sock01.index = [sock01.drawer.socks indexOfObject:sock01];

Answer 4

As far as I understand this functionality, it allows to have ordered Socks in every Drawer. As Apple writes in documentation:

You should use them only if a relationship has intrinsic ordering that is critical to its own representation—such as the steps in a recipe.

This means you can't fetch all Socks using sorted relation. Sorted Socks will be available only in every Drawer object.

Answer 5

To add some more clarity to the answer from adonoho (thanks mate) which helped me work it out - rather than trying to specify the to-many relationship as any sorting key which I also couldn't get working, specify the belongs-to relationship in a predicate to select which entities you want in the fetched results controller, and specify the belongs-to relationship as the sort key.

This satisfies the primary aim of having the results in a NSFetchedResultsController with all that goodness, and respects the order in the to-many relationship.

In this particular example (fetching socks by drawer):

// socks belong-to a single drawer, sort by drawer specified sock order
fetchRequest.sortDescriptors = @[[[NSSortDescriptor alloc] initWithKey:@"drawer" ascending:YES]];

// drawer has-many socks, select the socks in the given drawer
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"drawer = %@", drawer];

Essentially, this uses a given drawer to specify which socks should go into the NSFetchedResultsController, and the ordering as specified by the to-many relationship.

Looking at the generated SQL (annotated from my example using different entity names) via a core data SQL debug:

CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, ...fields..., t0.ZDRAWER, t0.Z_FOK_DRAWER FROM ZSOCKS t0 WHERE  t0.ZDRAWER = ?  ORDER BY  t0.Z_FOK_DRAWER

you can see the SQL ordering using the Z_FOK_DRAWER column, which Core Data uses for the position of that sock.

Answer 6

As I just answered here, I prefer just adding a new property to my NSManagedObject via a category.

Just add a method:

- (NSUInteger)indexInDrawerSocks
{
    NSUInteger index = [self.drawer.socks indexOfObject:self];
    return index;
}

Then in your NSFetchedResultsController, use sort descriptor:

fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"indexInDrawerSocks" ascending:YES]];