Question

I want to store NoteObjects in Core Data. Normally, a NoteObject has a NSString *mainText and an NSMutableArray *arrayOfTags (an array of NSStrings). I want to now use Core Data, but arrays are a tricky matter with core data. Typically a NoteObject won’t have more than 50 tags in its array. So how should I model this? I have two options:

1. Use a transformable property to store the array
2. Use a to-many relationship, which I’ve read is the more “legit” way to do it.

Which one should I use and why? And how would I implement a to-many relationship with my simple structure? I can’t seem to wrap my fingers around that concept.

Answer 1

Use to-many relationship. Because it’s way better and easier during fetch requests. See the screenshots below. Pay attention to the Relationship manager on the right side, set “To-Many Relationship” from your NoteObject to Tags. Ignore the Player entity.

Oh and pay attention to the “Delete Rule”. You might want to delete all the tags associated with a given NoteObject. So set it to Cascade in that case.

NoteObject entity

Tag entity

–Edit:

To add multiple tags you need to first fetch your NoteObject – I assume there will be some sort of ID parameter which you’ll use to distinguish NoteObjects. CoreData will automatically generate the add/remove methods for Tags. You’ll need to use code similar to the one below:

- (void)addTags:(NSArray *)tags toNoteObjectWithID:(NSString *)noteID {
  NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"NoteObject"];
  NSPredicate *pred = [NSPredicate predicateWithFormat:@"noteID == %@", noteID];
  [fetchRequest setPredicate:pred];
  NoteObject *noteObject = [[[self managedObjectContext] executeFetchRequest:fetchRequest error:nil] lastObject];

  for (NSString *tag in tags) {
    Tag *t = [NSEntityDescription insertNewObjectForEntityForName:@"Tag"
                                           inManagedObjectContext:[self managedObjectContext]];
    t.body = tag;
    t.noteObject = noteObject;
    [noteObject addTagsObject:t];
  }
  [self saveContext];
}