UICollectionView didChange and CoreData constraints - ios

Question

I have UICollectionView that linked to CoreData entity via NSFetchedResultsController.
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
if type == .insert {
blockOperations.append(BlockOperation(block: {
self.MessagesCollectionView?.insertItems(at: [newIndexPath!])
}))
}
if type == .update {
blockOperations.append(BlockOperation(block: {
self.MessagesCollectionView?.reloadItems(at: [newIndexPath!])
}))
}
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
let count = fetchedResultsControler.sections?[0].numberOfObjects
return count!
}
UICollection updates properly when i am inserting unique data into entity. But when i insert data that conflicts with unique constraint - new record in entity does not inserts (its ok), but UICollection didChange method gets '.insert' type of operation and inserts empty row!!! How to figure out this constraint issue in didChange method?
I can check existent of record in DB layer class before inserting, but in this case constraints are useless...
Here is the NSFetchResultController code:
lazy var fetchedResultsControler in UICollectionView: NSFetchedResultsController<ChatMessages> = {
let delegate = UIApplication.shared.delegate as! AppDelegate
let context = delegate.persistentContainer.viewContext
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "ChatMessages")
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "message_created", ascending : true)]
fetchRequest.includesPendingChanges = false
let frc = NSFetchedResultsController(fetchRequest: fetchRequest,
managedObjectContext: context,
sectionNameKeyPath: nil,
cacheName: nil )
frc.delegate = self
return frc as! NSFetchedResultsController<ChatMessages>
}()
Here is the core data insert code:
let newValue = NSEntityDescription.insertNewObject(forEntityName: "ChatMessages", into: context) as NSManagedObject
newValue.setValue(text, forKey: "text")
newValue.setValue(message_id, forKey:"message_id")
let parentContext = delegate.persistentContainer.viewContext
parentContext.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy
context.parent = parentContext
context.performAndWait {
do
{
dump(value)
try context.save()
print("Obj prepared for saving:")
print(value)
parentContext.performAndWait {
do {
try parentContext.save()
} catch {
print("Failture to save context: \(error)")
parentContext.rollback()
}
}
}
catch
{
print("Object exists...")
context.rollback()
}
}