Swift to-Many Relationship .removeFromMethod() doesn't update NSFetchedResultsController

Question

Main Objective: In first view controller I shall tap on a button in a cell -> button calls entity.removeFrom" "(_ value: " " ) method -> automatically should remove the corresponding cell from a table view in a second view controller.

What is Occurring: " " ... entity.removeFrom" "(_ value: " " ) method -> object is removed, set to nil, however NSFetchedResultsController is not deleting the respective rows. Leaving the tableview filled with empty cells ( from the remaining nil objects )

Question: How to remove many object in the to-many relationship and update the NSFetchedResultsController accordingly.

About the app Using swift 3 I am creating a basic To-Do list app with a tab bar controller.

First tab = A tableView of ToDoItem

Second tab = Finished toDoItems in an NSFetchedResultsController

Core Data Entities: FinishedCollection, ToDoItem

FinishedCollection has a one-to-many relationship with todoItems (destination is ToDoItem)

Order of operation:

First Tab

1) Set ToDoItem attribute isFinished

2) FinishedCollection.addToToDoItems(toDoItem) or FinishedCollection.removeFromToDoItems(toDoItem)

3) Automatically Update Second Tab NSFetchedResultsController

PROBLEM: When I removeFromToDoItems(toDoItem) the property in the NSFetchedResults controller is set to nil rather than removing the object entirely( I don't necessarily want to remove the object entirely) Resulting in empty cells and a million-frazillion nil objects. How do I approach removing the many nil objects in the one-to-many relationship and only having finished toDoItems in the NSFetchedResults controller tableview tab.

-

Attempt to filter nil objects when setting numberOfRowsInSection tableview method. Does not seem to do anything in this circumstance

let notNilObjects = sectionInfo.objects.flatMap { 0 }

TableViewDataSource methods

  func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    let sections    = fetchedResultsController?.sections
    let sectionInfo = sections?[section]

    return sectionInfo?.numberOfObjects ?? 1
  }

  func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell( withIdentifier: "JournalCell", for: indexPath )

    configure(cell, atIndexPath: indexPath)

    return cell
  }

//NSFetchedResultsDelegate methods

fileprivate func configure(_ cell: UITableViewCell, atIndexPath indexPath: IndexPath) {
  guard let selectedObject = fetchedResultsController?.object(at: indexPath) else {
    fatalError("Unexpected Object in FetchedResultsController")
  }

  let toDoItems = selectedObject.dayTasks?.allObjects as! [ToDoItem]
  let toDo = ToDoItems.last

  print(toDo?.name)
}

func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
  tableView.beginUpdates()
}

func controller(_ controller               : NSFetchedResultsController<NSFetchRequestResult>,
                didChange sectionInfo      : NSFetchedResultsSectionInfo,
                atSectionIndex sectionIndex: Int,
                for type                   : NSFetchedResultsChangeType) {
  switch type {
  case .insert:
    tableView.insertSections(NSIndexSet(index: sectionIndex) as IndexSet, with: .fade)
  case .delete:
    tableView.deleteSections(NSIndexSet(index: sectionIndex) as IndexSet, with: .fade)
  case .move:
    break
  case .update:
    tableView.reloadData()
  }
}

func controller(_ controller      : NSFetchedResultsController<NSFetchRequestResult>,
                didChange anObject: Any,
                at indexPath      : IndexPath?,
                for type          : NSFetchedResultsChangeType,
                newIndexPath      : IndexPath?) {
  switch type {
  case .insert:
    tableView.insertRows(at: [newIndexPath!], with: .fade)
  case .delete:
    tableView.deleteRows(at: [indexPath!], with: .fade)
  case .update:
    configure(tableView.cellForRow(at: indexPath!)!, atIndexPath: indexPath!)
  case .move:
    tableView.moveRow(at: indexPath!, to: newIndexPath!)
  }
}

func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
  tableView.endUpdates()
}


func initializeFetchedResultsController() {
  let request: NSFetchRequest = ToDoItem.fetchRequest()
  request.sortDescriptors = [ NSSortDescriptor( key: "timestamp", ascending: true ),
                              NSSortDescriptor( key: "isAccomplished", ascending: true) ]

 fetchedResultsController = NSFetchedResultsController( fetchRequest        : request,
                                                       managedObjectContext: CoreDataHelper.context,
                                                       sectionNameKeyPath  : nil,
                                                       cacheName           : nil )
  fetchedResultsController?.delegate = self

  do {
    try fetchedResultsController?.performFetch()
  }
  catch {
    print("Couldn't fetch results controller")
  }
}

Answer 1

In your initializeFetchedResultsController method add

request.predicate = NSPredicate(format: "isFinished = 1")

Answer 2

In order to filter an NSSet acquired from a fetchedResultsController one must use an NSPredicate and assign it to the request.predicate property of an NSFetchRequest

Example

let request = Entity.fetchRequest()

request.predicate = NSPredicate(format: "isAccomplished == YES")