Error when working with NSManagedObjectContext in background thread - swift

Question

So I keep getting this error when I save to core data.
Coredata[21468:13173906] [error] error: SQLCore dispatchRequest: exception handling request: , ** -_referenceData64 only defined for abstract class. Define -[NSTemporaryObjectID_default _referenceData64]! with userInfo of (null)
I recently finished a course on core data, the instructor brush a bit on the topic that core data isn't thread safe. So what he suggested is a child/parent context, so that is what I tried doing but kept getting the error from above. This is how my code looks.
struct Service {
static let shared = Service()
func downloadPokemonsFromServer(completion: #escaping ()->()) {
let urlString = "https://pokeapi.co/api/v2/pokemon?limit=9"
guard let url = URL(string: urlString) else { return }
URLSession.shared.dataTask(with: url) { (data, response, error) in
if let err = error {
print("Unable to fetch pokemon", err)
}
guard let data = data else { return }
let privateContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
privateContext.parent = CoreDataManager.shared.persistentContainer.viewContext
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
do {
let pokemonJSON = try decoder.decode(PokemonsJSON.self, from: data)
pokemonJSON.pokemons.forEach { (JSONPokemon) in
//Works fine with privateContext here
let pokemon = Pokemon(context: privateContext)
pokemon.name = JSONPokemon.name
pokemon.url = JSONPokemon.detailUrl
}
//Why does the privateContext work here
//and doesn't crash my app.
try privateContext.save()
try privateContext.parent?.save()
completion()
} catch let err {
print("Unable to decode PokemonJSON. Error: ",err)
completion()
}
}.resume()
}
func fetchMoreDetails(pokemon: Pokemon, urlString: String, context: NSManagedObjectContext, completion: #escaping ()->()) {
guard let url = URL(string: urlString) else { return }
URLSession.shared.dataTask(with: url) { (data, response, error) in
if let err = error {
print("Unable to get more details for pokemon", err)
}
guard let data = data else { return }
let privateContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
privateContext.parent = CoreDataManager.shared.persistentContainer.viewContext
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
do {
let pokemonDetailJSON = try decoder.decode(PokemonDetailJSON.self, from: data)
pokemonDetailJSON.types.forEach { (nestedType) in
// I can just change this to context and it works
// Why doesnt it work with privateContext and crashes my app
let type = Type(context: privateContext)
type.name = nestedType.type.name
pokemon.addToTypes(type)
}
try privateContext.save()
try privateContext.parent?.save()
completion()
} catch let err {
print("Unable to decode pokemon more details", err)
completion()
}
}.resume()
}
}
So in the downloadPokemonsFromSever, I make a call to an api that parse the json and saves it into Coredata. I followed my instructor instructions, by creating a privateContext and then setting its parent to my mainContext. Then I create a new Pokemon that has a name & url using my privateContext and NOT my mainContext. When I completely parse my Pokemon I go into another api that has more details on that Pokemon.
This is where my app starts to crash. As you can see from the fetchMoreDetails there is a parameter that is context. When I try to create a new Type with privateContext it crashes my app. When I use the context that is passed through it works fine. I would like to know why privateContext works inside downloadPokemonFromServer and not in fetchMoreDetails. I left a comment above the line that I think that crashes my app. This is how I call it in my ViewController, using this action.
#objc func handleRefresh() {
Service.shared.downloadPokemonsFromServer {
let context = CoreDataManager.shared.persistentContainer.viewContext
self.pokemonController.fetchedObjects?.forEach({ (pokemon) in
Service.shared.fetchMoreDetails(pokemon: pokemon, urlString: pokemon.url ?? "", context: context) {
}
})
}
tableView.refreshControl?.endRefreshing()
}