How to store array of NSManagedObjects in an NSManagedObject - iphone

Question

I am loading my app with a property list of data from a web site. This property list file contains an NSArray of NSDictionaries which itself contains an NSArray of NSDictionaries. Basically, I'm trying to load a tableView of restaurant menu categories each of which contains menu items.
My property list file is fine. I am able to load the file and loop through the nodes structure creating NSEntityDescriptions and am able to save to Core Data. Everything works fine and expectedly except that in my menu category managed object, I have an NSArray of menu items for that category. Later on, when I fetch the categories, the pointers to the menu items in a category is lost and I get all the menu items. Am I suppose to be using predicates or does Core Data keep track of my object graph for me?
Can anyone look at how I am loading Core Data and point out the flaw in my logic? I'm pretty good with either SQL and OOP by themselves, but am a little bewildered by ORM. I thought that I should just be able to use aggregation in my managed objects and that the framework would keep track of the pointers for me, but apparently not.
NSError *error;
NSURL *url = [NSURL URLWithString:#"http://foo.com"];
NSArray *categories = [[NSArray alloc] initWithContentsOfURL:url];
NSMutableArray *menuCategories = [[NSMutableArray alloc] init];
for (int i=0; i<[categories count]; i++){
MenuCategory *menuCategory = [NSEntityDescription
insertNewObjectForEntityForName:#"MenuCategory"
inManagedObjectContext:[self managedObjectContext]];
NSDictionary *category = [categories objectAtIndex:i];
menuCategory.name = [category objectForKey:#"name"];
NSArray *items = [category objectForKey:#"items"];
NSMutableArray *menuItems = [[NSMutableArray alloc] init];
for (int j=0; j<[items count]; j++){
MenuItem *menuItem = [NSEntityDescription
insertNewObjectForEntityForName:#"MenuItem"
inManagedObjectContext:[self managedObjectContext]];
NSDictionary *item = [items objectAtIndex:j];
menuItem.name = [item objectForKey:#"name"];
menuItem.price = [item objectForKey:#"price"];
menuItem.image = [item objectForKey:#"image"];
menuItem.details = [item objectForKey:#"details"];
[menuItems addObject:menuItem];
}
[menuCategory setValue:menuItems forKey:#"menuItems"];
[menuCategories addObject:menuCategory];
[menuItems release];
}
if (![[self managedObjectContext] save:&error]) {
NSLog(#"An error occurred: %#", [error localizedDescription]);
}

Answer 1

You set a NSArray as to-many relationship object
NSMutableArray *menuItems = [[NSMutableArray alloc] init];
[menuCategory setValue:menuItems forKey:#"menuItems"];
which might cause the trouble.(should throw an exception?) Relationships in CoreData are always unsorted, therefore NSSets. Add a sortIndex property to your entities for ordering.

Answer 2

I had the same issue. There are 2 major problems with using NSSets and Core Data: if you need non-distinct objects and need them ordered. As an example, say you have 2 entities in Core Data: professor and student. The student takes 10 classes for a degree program and you wish to have a (one-to-many) relationship from the student to the professor in order that the classes were taken. Also, the same professor may teach more than one class. This was how I overcame the issue. Create a Binary Data attribute (we'll call it profData) in student and store dictionaries that make it possible to reconstruct the data as needed. Note: don't store an array of professors, since they inherit from NSManagedObject vs. NSObject. That can cause problems. You can bolt on the required methods using a category. In this example, I created a category on Student called ProfList (Student+ProfList.h/m). This keeps the code out of the NSManagedObject subclasses, so if my attributes in Core Data change, I can regenerate the subclasses automatically without wiping out this code. Here is some sample code:
// Student+ProfList.h
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#import "Student.h"
#import "Professor.h"
#interface Student (ProfList)
- (NSArray *)getStudentsFullList;
- (void)storeStudentsFullList:(NSArray *)fullList;
#end
// Student+ProfList.m
#import "Student+ProfList.h"
#implementation Student (ProfList)
- (NSArray *)getStudentsFullList
{
NSData *storedData = self.profData;
if (!storedData) return nil;
NSMutableArray *fullList = [[NSMutableArray alloc] init];
// Retrieve any existing data
NSArray *arrayOfDictionaries = [NSKeyedUnarchiver unarchiveObjectWithData:storedData];
// Get the full professor list to pull from when recreating object array
NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:#"Professor"];
NSSortDescriptor *alphaSort =
[[NSSortDescriptor alloc] initWithKey:#"name"
ascending:YES
selector:#selector(localizedCaseInsensitiveCompare:)];
[fetchRequest setSortDescriptors:#[alphaSort]];
NSSet *allProfessors = [NSSet setWithArray:[context executeFetchRequest:fetchRequest error:nil]];
for (NSDictionary *dict in arrayOfDictionaries) {
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"name LIKE %#", [dict objectForKey:#"name"]];
NSSet *filteredSet = [allProfessors filteredSetUsingPredicate:predicate];
Professor *newProfessor = [filteredSet anyObject];
newProfessor.index = [dict objectForKey:#"index"];
[fullList addObject:newProfessor];
}
return fullList;
}
- (void)storeStudentsFullList:(NSArray *)fullList
{
NSMutableArray *encodedList = [[NSMutableArray alloc] init];
for (Professor *professor in fullList) {
[encodedList addObject:#{#"index" : #([encodedList count]), #"name" : professor.name}];
}
NSArray *encodedArray = [NSArray arrayWithArray:encodedList];
NSData *arrayData = [NSKeyedArchiver archivedDataWithRootObject:encodedArray];
self.profData = arrayData;
}
#pragma mark - Core Data
- (NSManagedObjectContext *)managedObjectContext
{
NSManagedObjectContext *context = nil;
id delegate = [[UIApplication sharedApplication] delegate];
if ([delegate performSelector:#selector(managedObjectContext)]) {
context = [delegate managedObjectContext];
}
return context;
}
#end
You store a local variable in a view controller, then send this message to the student instance to get the list and save it locally for use in a table view or whatever.