Coredata fetching and grouping objects in sections - ios

Question

I am working with a pretty complex object model and am having a bit of trouble with breaking some of my fetches down into sections to display in a tableview.
I have the need to group Meeting managed objects into a few different "pockets" such as project, client and a few others. For several reasons I've decided to implement these as tags that can be associated with a Meeting entity.
So I have created a new Tag entity which has a type and a value and established the relationships between the two:
Meeting <<-->> Tag
If I want to associate a Meeting with a project, I create a Tag with name 'project' and value 'Project Name', then add it to the Meeting entity via the relationship.
I initially thought about using a NSFetchedResultsController but I am getting all sorts of issues all of which I don't really understand.
For instance, this fetch (I'm omitting the unnecessary bits):
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:[Meeting entityName] inManagedObjectContext:moc];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"tags.name contains[] 'client'"];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"title" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
[fetch setEntity:entity];
[fetch setPredicate:predicate];
[fetch setSortDescriptors:sortDescriptors];
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch
managedObjectContext:moc
sectionNameKeyPath:#"self.tags.value"
cacheName:nil];
In this particular case the fetch does work but somehow I am getting unexpected results where not only Tags with value client are presented but also ones with value project???
If I change my predicate to tags.name == 'project' I get an exception:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'
I'm probably missing something basic here and admittedly I don't have a lot of experience with predicates but Apple's documentation on the subject leaves a lot to be desired.
As a side question and something that I don't understand either is why do I have to add self to my sectionNameKeyPath in self.tags.value? What does it do?? In this case if I don't add it I get an exception thrown as well:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid to many relationship in setPropertiesToFetch: (tags.value)
Finally, what is the alternative to using fetched results controller in this case? Will it be a bunch fetch requests where I first get each instance of Tag where name == 'project' and iterate through the array to pull the Meeting objects associated with it? It seems highly inefficient but all I can think of at the moment so if you have any other ideas I am very interested in hearing them.
Many thanks in advance for your time,
Rog

Answer 1

The issue is that Meeting has-many tags, so you're going to need to use aggregate operations:
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:[Meeting entityName] inManagedObjectContext:moc];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"ANY tags.name contains[cd] 'client'"];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"title" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
[fetch setEntity:entity];
[fetch setPredicate:predicate];
[fetch setSortDescriptors:sortDescriptors];
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch
managedObjectContext:moc
sectionNameKeyPath:#"clientName"
cacheName:nil];
i.e. Give me a list of all Meeting objects where ANY of the tags are of type client, and group them by clientName. For the clientName key path to work, you'll need to implement a transient property:
- (NSString *)clientName {
[self willAccessValueForKey:#"clientName"];
// Set clientName to the value of the first tag with name 'client'
NSString* clientName = #"...";
[self didAccessValueForKey:#"clientName"];
return clientName;
}
If a number of your NSManagedObject subclasses need the clientName property, you can implement it in a common abstract NSManagedObject subclass and make your concrete subclasses inherit from that.

Answer 2

"Apple's documentation on [NSPredicate] leaves a lot to be desired" - totally agreed!
tags.name is not valid because tags is a collection, there's no object, there are (let's say) three of them
i think you want something like "tags contains %#", projectTag but i'm not sure of the syntax. might be "%# IN %#", projectTag, thing.tags