Core data Fetch request on relationship properties - swift

Question

My app has a number of wordLists each of which contain a number of words. In one view controller a tableView lists the wordLists and then a sub view controller has a tableView with the words in it. The segue passes the wordList entity. But I cannot work out how to do a fetch request on the wordList passed to then get all the words. Error message is shown below. I need to do a fetch request rather than looking at properties so I can do a sort.
Thanks in advance for any help on this one....
The WordList entity has an attribute listName, and relationships: words, destination: Word, Inverse: wordList
The Word entity has an attribute wordName, wordIndex and relationships: wordList, destination: WordList, Inverse: words
My WordsViewController looks like:
class WordsViewController: UIViewController, UITableViewDataSource, UITableViewDelegate, UITextFieldDelegate {
var coreDataStack: CoreDataStack = (UIApplication.sharedApplication().delegate as! AppDelegate).coreDataStack
var wordList: WordList?
var words = [Word]()
var word: Word?
override func viewDidLoad() {
super.viewDidLoad()
// set the title of the scene
title = wordList?.listName
// fetch all the words that are part of the wordList passed to this VC.
let fetchRequest = NSFetchRequest(entityName: "Word")
let wordListPredicate = NSPredicate(format: "word.wordList == '\(wordList)'") // GIVES AN ERROR SAYING UNABLE TO PARSE THE FORMAT STRING "word.wordList == 'Optional(<WordList:0x...
let sortDescriptor = NSSortDescriptor(key: "wordIndex", ascending: true)
fetchRequest.sortDescriptors = [sortDescriptor]
fetchRequest.predicate = wordListPredicate
do {
if let results = try coreDataStack.managedObjectContext.executeFetchRequest(fetchRequest) as? [Word] {
words = results
}
} catch {
fatalError("There was an error fetching system person")
}
}

Answer 1

You cannot use string interpolation to build a predicate, use
argument substitution instead. In your case (since var wordList: WordList? is an
optional):
if let theWordlist = wordlist {
let wordListPredicate = NSPredicate(format: "wordList == %#", theWordlist)
} else {
// wordlist is nil ...
}
Note also that "word.wordList == " in the predicate should be
"wordList == ", because "wordList" is the property of the "Word"
entity.
For more information, see Predicate Format String Syntax
in the "Predicate Programming Guide".

Answer 2

You should use a NSFetchedResultsController, not fetch all data in viewDidLoad. The fetched results controller will help you fetch and display the data efficiently, react to changes, optimize memory, etc.
In the fetched results controller's fetch request (which fetches entity Word), use this predicate (with a safeguard if your optional wordList is nil):
request.predicate = wordList != nil ?
NSPredicate(format: "wordlist = %#", wordList!) :
NSPredicate(value: false)
This assumes that there is a one-to-many relationship between word list and its words. I would recommend this setup, even if it implies that you might have to store the same word more than once if it is in different lists.
NB: the false predicate ensures you get a valid fetch request which will fetch nothing.