I am kind of performing an upsert (update or insert) operation after fetching from an JSON API for some entities which have many to one relations.

Eg: Sections contains Categories, Categories contains Songs
So while going through a for loop I kind of get a crash while performing operation.

This is my fetch logic using a predicate.

public extension SongEntity {

    static func fetchOrCreate(for songID: String, in context: NSManagedObjectContext) -> SongEntity {
        PersistenceController.shared.fetchSingle(SongEntity.self, predicate: SongEntity.bySongID(songID)) ?? SongEntity(context: context)
    }

    static func bySongID(_ songID: String) -> NSPredicate {
        NSPredicate(format: "identifier == %@", songID)
    }

}

public func fetchSingle<T: NSManagedObject>(
  _ entityClass: T.Type,
  sortBy sortDescriptors: [NSSortDescriptor]? = nil,
  predicate: NSPredicate? = nil
) -> T? {
  let entityName = String(describing: entityClass)
  let fetchRequest = NSFetchRequest<T>(entityName: entityName)

  fetchRequest.sortDescriptors = sortDescriptors
  fetchRequest.predicate = predicate
  fetchRequest.fetchLimit = 1 

  do {
    let items = try PersistenceController.shared.container.viewContext.fetch(fetchRequest)
    return items.first
  } catch {
    debugPrint(error.localizedDescription)
    return nil
  }
}

This is where the crash tends to appear

for section in songsData.sections {
  let sectionEntity = Section.fetchOrCreate(for: section.identifier, in: viewContext)
  sectionEntity.identifier = section.identifier
  sectionEntity.title = section.title
  var categories: [Category] = []
  for category in songsData.sectionCategories {
    if category.sectionID == section.identifier {
      let existingCategory = Category.fetchOrCreate(for: category.identifier, in: viewContext)
      existingCategory.title = category.title
      existingCategory.identifier = category.identifier
      existingCategory.sectionID = category.sectionID
      var songs: [SongEntity] = []
      for song in songsData.songs {
        if song.categoryID == category.identifier {
          // CRASH OCCURS HERE
          let existingSong = SongEntity.fetchOrCreate(for: song.identifier, in: viewContext)
          existingSong.categoryID = song.categoryID
          existingSong.identifier = song.identifier
          existingSong.title = song.title
          songs.append(existingSong)
        }
      }
      existingCategory.addToSongs(NSOrderedSet(array: songs))
      categories.append(existingCategory)
    }
  }
  sectionEntity.addToCategories(NSOrderedSet(array: categories))
}

What am I doing wrong? Am I handling the relationships correctly?

Sometimes I also get EXC_BAD_ACCESS error as well? I am novice to relationships upserts. So please help me out where I am wrong exactly