Question

I am a noob when it comes to traversing Core Data many-to-many relationships (I have read numerous threads and documentation on this so unless it’s and end all, please no links to documentation or threads).

I am making an inventory application and currently have a Core Data model that includes an “Thing”, “Category”, “Location”, and “ThingLocation” (ThingLocation is an entity that holds both a Thing and Location reference but includes the amount of Things on that particular Location. Also a many-to-many relationship) Entities that I would like to populate my UI with. I am proficient in GUI so this is not a question of User Interface but rather how I would gather the information using (probably) NSPredicates.

Ex: If I show a TableView consisting of a Category entity’s details then how would I populate it with the Things in that Category Entity.

Ex: If I wanted to display a UILabel showing the total amount of Thing‘s there were in it. (i.e. add up all of the amounts on each Location).

EDIT: I want to be able to use an NSFetchedResultsController!

Answer 1

I am not exactly sure what your question is asking. So for example you want to iterate over all the categories and all the things in that category you would first do a request for the entities of category without a predicate (this will return all category objects) and iterate over all those with fast enumeration:

//iOS 5 way of doing it
NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntity:@"Category"];
NSArray* arrayOfObjects = [context executeFetchRequest: request withError: nil];
for (Category* cat in arrayOfObjects)
{
    //iterate over all the things in that category
    for (Thing* thing in cat.things){
    {
        //do something?
    }
}

For your first example of populating a tableview with things in a category,

If you have the category you would get the Things very easily like this:

NSSet* things= category.things;
//you can get it into an array by sorting it somehow or just get it like that.
NSMutableArray* things = [[category.things allObjects] mutableCopy];

You can iterate over this in a very normal fashion or use them as your datasource for your tableview. If you don’t have the category you need something to distinguish it in which case you would set up the predicate like this:

NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntity:@"Thing"];
NSPredicate* pred = [NSPredicate predicateWithFormat:@"(relationToCat.DestAtt = %@)",howToDestinguish];

This will return all the Things that are connected to a category that has that attribute.

For your second example, You would set the NSPredicate up to get all the ThingLocations for that specific Thing. Then iterate over them and add up the values at the locations. If you wanted to do this for every category over everything it would just require you to nest some for loop starting with the categories. then for each thing get all the ThingLocations and for each of those add up the values.

I hope that answers your questions. To-Many relations are just sets and can be treated as such. I find that thinking from the bottom up helps me form the predicates. thinking I need all the things in this category so I would set up with the entity of Things and connecting it back to the category in the predicate.

Edit: NSFetchedResultsController example
In your .h file after declaring your super class add NSFetchedResultsControllerDelegate to the delegates implemented.

Create an ivar:
@property (nonatomic, retain) NSFetchedResultsController* fetchedResultsController;

On the implementation side I’ve seen two different approaches, the first is writing a custom accessor for the property that initializes it there, the other is just to do it in the viewDidLoad in either case the setup is as follows:

NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Thing" inManagedObjectContext:context];
NSPredicate* pred=[NSPredicate predicateWithFormat:@"(relToCat.something=%@)",something];
[fetchRequest setPredicate:pred];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:20]; //tells it how many objects you want returned at a time.

//this is for displaying the things in some sort of order. If you have a name attribute you'd do something like this
NSSortDescriptor *descriptor = [[[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES] autorelease];
NSArray *sortDescriptors = [[[NSArray alloc] initWithObjects: descriptor, nil] autorelease];
[fetchRequest setSortDescriptors:sortDescriptors];

//this is the set up. If you put a sectionNameKeyPath it will split the results into sections with distinct values for that attribute
NSFetchedResultsController *frc = nil;
frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context] sectionNameKeyPath:@"attribute that splits it into sections" cacheName:nil];
[frc setDelegate:self];
[self setFetchedResultsController:frc];
[frc release];
frc = nil;

//Tells it to start.
[fetchedResultsController performFetch:nil];

Then for the table view delegate methods it is a piece of cake like so:

-(NSString*)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section{

    return [[[fetchedResultsController sections] objectAtIndex:section] name];
}
- (NSInteger)numberOfSectionsInTableView:(UITableView*)tableView {
        return [[fetchedResultsController sections] count];
}
- (NSInteger)tableView:(UITableView*)tableView numberOfRowsInSection:(NSInteger)section
{
        return [[[fetchedResultsController sections] objectAtIndex: section] numberOfObjects];
}
/* If you want the bar on the right with the names of the sections...
-(NSArray*) sectionIndexTitlesForTableView:(UITableView *)tableView{
    return [fetchedResultsController sectionIndexTitles];
}*/
-(UITableViewCell* )tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
        static NSString* ident=@"cellident";
    UITableViewCell *cell = [self.itemTable dequeueReusableCellWithIdentifier:ident]; 
    if (!cell) {
        cell = [[[UITableViewCell alloc] initWithStyle: UITableViewCellStyleDefault reuseIdentifier:ident] autorelease];
    }
    [self configureCell:cell atIndexPath:indexPath];
    return cell;
}
- (void) configureCell:(UITableViewCell*)cell atIndexPath:(NSIndexPath*)indexPath {
    NSManagedObject* item=[fetchedResultsController objectAtIndexPath:indexPath];
    //set up your cell somehow
    return cell;
}

You also need to add the delegate methods for the fetched results controller. They are all very simple and look something like this:

- (void)controllerWillChangeContent:(NSFetchedResultsController*)controller {
    [tableView beginUpdates];
}
- (void)controller:(NSFetchedResultsController*) controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    NSIndexSet *set = [NSIndexSet indexSetWithIndex:sectionIndex];
    switch(type) {
        case NSFetchedResultsChangeInsert: [tableView insertSections:set  withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [tableView deleteSections:set withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}
- (void)controller:(NSFetchedResultsController*)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath*)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath*)newIndexPath
{
    UITableView *tv = tableView;
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [tv insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;
        case NSFetchedResultsChangeDelete:
            [tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;
        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tv cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;
        case NSFetchedResultsChangeMove:
            [tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tv insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }

}
- (void)controllerDidChangeContent:(NSFetchedResultsController*)controller {
    [tableView endUpdates];
}

This will update the table if in some other part of the program a thing is added it will automatically show up on the table if the predicate matches.