Need help with many-to-many relationships in core data for iPhone

Question

I have come to a roadblock in my current project. I basically have an app that is much like the Core Data Recipe app... Here is the basic structure I have in my .xcdatamodel

Entity: Restaurant String: name Category: category <---- to-many relationship

Entity: Category String: name Restaurant: restaurant <---- to-many relationship So basically, a Restaurant can have multiple categories... And there are a const number of predefined Categories.. For example: Restaurant: Name: Chili’s Categories: Take Out , Family Dining

“Take Out” and “Family Dining” are 2 of 10 different possible Restaurant Categories. How do I go about doing this.. I have looked at the sqllite database and I have my ZRestaurant and ZCategory table + the join table for both of them... I have data in both...

How do I go about setting my Restaurants Catagory with the different values? and then how do I fetch them back?

Thanks all! Kurt

Answer 1

OK, After working on this for the past 2 days I finally came up with my solution which was actually a mix between Alex and Wills suggestions... Thank you to both of you!!

Here is what I have...

 NSManagedObjectContext *context = [restaurant managedObjectContext];


NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Category" inManagedObjectContext:context]];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:&sortDescriptor count:1];
[fetchRequest setSortDescriptors:sortDescriptors];

NSError *error = nil;
NSArray *possibleCategories = [context executeFetchRequest:fetchRequest error:&error];

categoryArray = [[NSMutableArray alloc] initWithArray:possibleCategories];

currentCategories = [restaurant valueForKeyPath:@"categories"];

[restaurant addCategoriesObject:(Category *)[possibleCategories objectAtIndex:15 ]];

[currentCategories addObject:(Category*)[categoryArray objectAtIndex:15]];

and then I save like this

- (void)save{
    NSLog(@"EditCatagoriesTableViewController - save");

    NSSet* myCategorySet = [[NSSet alloc] initWithSet:currentCategories];

    NSError *error = nil;

    [restaurant addCategories:myCategorySet];


    error = nil;
    if (![restaurant.managedObjectContext save:&error]) {
        // Handle error
        NSLog(@"restaurant - Unresolved error %@, %@", error, [error userInfo]);
        exit(-1);  // Fail
    }       
}

And that does it!

Thank you so much for the help you two!!!

-Kurt