display DetailsViewController using NSFetchedResultController

Question

Hi everyone,

In my iphone app I have core data to-many relation topic -->> question, so every topic contains a few questions, I figured out how to display list of topics in UItableView using NSFetchedResultController, but I can't get how shall I construct my FetchedResultController to display questions in chosen topic.

I'm trying to find on Internet similar tutorial or example but can't find anything.

How can I do it?

Thank you very much

Answer 1

You won't use an NSFetchedResultsController. Instead you'll define a property in your detail view controller

@property (nonatomic, retain) Topic *topic;

Then when you create your detail view controller, you'll set the property based on the topic selected in didSelectRowForIndexPath.

Then in your detail view it's just a matter of grabbing the questions out of the Topic object using the core data relationship (topic.questions)

Answer 2

Ok, I found out that I need to use NSPredicate for it. Now I'm trying to add it to my fetchedResultController like this:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"topic == %@", self.topic];
[fetchRequest setPredicate:predicate];

but getting an error:

'keypath topic not found in entity <NSSQLEntity QuestionItem id=2>'

How can I do it?

Thanks

Edit

Got it!

need to set up both side relation in data model and set predicate like this:

[NSPredicate predicateWithFormat:@"topic.topicName == %@", self.topic]

Answer 3

You do NOT need to go back to a NSFetchRequest to get the objects on the other end of a relationship. Just ask your Topic for its questions. This will give you a NSSet of entities that are faulted. Faulted entities take up VERY little memory and you can pull in 1000 of them easily.

If you need to filter those then you can perform a -filteredSetWithPredicate: on the NSSet that is returned but going out to disk is wasteful and slow.