Issue using CoreData with Codable protocol - swift

Question

I created two NSManagedObject classes one for Songs and one for Categories of each song. And they have a one to many relationship. What i do is that i download a json file from the server and parse it using Decodable and save the data in CoreData. Every thing is smooth except when i try to add songs to a certain category type i get a crash.
'Illegal attempt to establish a relationship 'category' between objects in different contexts
I know what this crash is and i know i have two context one for the category class and one for the song class. The issue is that tutorials for CoreData using Decodable is so little. So now i am thinking of a way maybe i can create a parent class of these classes and init the context in it and just call super.init() in the subclasses of category and songs. But i really cannot do it. Or maybe there is a much simpler way. I will share the code of my classes here and the code where the error is happening.
struct CategoryData: Decodable {
let data: [CategoryManagedObject]
}
#objc(CategoryManagedObject)
class CategoryManagedObject: NSManagedObject, Decodable {
// MARK: - Core Data Managed Object
#NSManaged var id: Int
#NSManaged var name: String
#NSManaged var imgUrl: String
#NSManaged var coverPhotoBit64: String
#NSManaged var jsonUrl: String
#NSManaged var version: Int
#NSManaged var order: Int
#NSManaged var songs: NSSet?
//var coreDataStack: CoreDataManager!
enum CodingKeys: String, CodingKey {
case name, coverPhotoBit64, id, jsonUrl, version, order
case imgUrl = "coverPhoto"
}
// MARK: - Decodable
required convenience init(from decoder: Decoder) throws {
//try super.init(from: decoder, type: "Categories")
guard let codingUserInfoKeyManagedObjectContext = CodingUserInfoKey.context,
let managedObjectContext = decoder.userInfo[codingUserInfoKeyManagedObjectContext] as? NSManagedObjectContext,
let entity = NSEntityDescription.entity(forEntityName: "Categories", in: managedObjectContext) else {
fatalError("FALIED TO DECODE CATEGORIES")
}
self.init(entity: entity, insertInto: managedObjectContext)
let container = try decoder.container(keyedBy: CodingKeys.self)
id = try container.decode(Int.self, forKey: .id)
name = try container.decode(String.self, forKey: .name)
imgUrl = try container.decode(String.self, forKey: .imgUrl)
coverPhotoBit64 = try container.decode(String.self, forKey: .coverPhotoBit64)
version = try container.decode(Int.self, forKey: .version)
jsonUrl = try container.decode(String.self, forKey: .jsonUrl)
order = try container.decode(Int.self, forKey: .order)
// if let sArray = songs.allObjects as? [Song] {
// songs = try container.decode(sArray.self, forKey: .song)
// }
}
#nonobjc public class func fetchRequest() -> NSFetchRequest<CategoryManagedObject> {
return NSFetchRequest<CategoryManagedObject>(entityName: "Categories")
}
}
public extension CodingUserInfoKey {
// Helper property to retrieve the context
static let context = CodingUserInfoKey(rawValue: "managedObjectContext")
}
// MARK: Generated accessors for songs
extension CategoryManagedObject {
#objc(addSongsObject:)
#NSManaged public func addToSongs(_ value: Song)
#objc(removeSongsObject:)
#NSManaged public func removeFromSongs(_ value: Song)
#objc(addSongs:)
#NSManaged public func addToSongs(_ values: NSSet)
#objc(removeSongs:)
#NSManaged public func removeFromSongs(_ values: NSSet)
}
#objc(Song)
public class Song: NSManagedObject, Decodable {
#NSManaged var id: Int
#NSManaged var name: String
#NSManaged var artist: String
#NSManaged var code: String
#NSManaged var category: CategoryManagedObject
enum CodingKeys: String, CodingKey {
case name, id, artist, code
}
// MARK: - Decodable
required convenience public init(from decoder: Decoder) throws {
guard let codingUserInfoKeyManagedObjectContext = CodingUserInfoKey.context,
let managedObjectContext = decoder.userInfo[codingUserInfoKeyManagedObjectContext] as? NSManagedObjectContext,
let entity = NSEntityDescription.entity(forEntityName: "Songs", in: managedObjectContext) else {
fatalError("FALIED TO DECODE CATEGORIES")
}
self.init(entity: entity, insertInto: managedObjectContext)
let container = try decoder.container(keyedBy: CodingKeys.self)
id = try container.decode(Int.self, forKey: .id)
name = try container.decode(String.self, forKey: .name)
artist = try container.decode(String.self, forKey: .artist)
code = try container.decode(String.self, forKey: .code)
}
#nonobjc public class func fetchRequest() -> NSFetchRequest<Song> {
return NSFetchRequest<Song>(entityName: "Songs")
}
}
This is where the crash happens because of two different context.
func saveJsonSongsInDB(filename fileName: String, category: CategoryManagedObject) {
do {
let data = try Data(contentsOf: URL(string: fileName)!)
//let context = CoreDataManager.shared.persistentContainer.newBackgroundContext()
let decoder = JSONDecoder()
decoder.userInfo[CodingUserInfoKey.context!] = dbContext
//decoder.userInfo[CodingUserInfoKey.deferInsertion] = true
coreDataStack.deleteAllRecords("Songs")
let songs = try decoder.decode([Song].self, from: data)
let s = NSSet(array: songs)
// category.managedObjectContext?.insert(<#T##object: NSManagedObject##NSManagedObject#>)
// dbContext.insert(category)
//print("SONGS: \(songs)")
category.addToSongs(s) //----> CRASH
try dbContext.save()
} catch let err {
print("error:\(err)")
}
}

Answer 1

First of all use one context, the context passed in the JSONDEcoder
In CategoryManagedObject declare songs as non-optional native type
#NSManaged var songs: Set<Song>
Decode songs as Set (yes, this is possible) and set the category of each song to self
songs = try container.decode(Set<Song>.self, forKey: .song)
songs.forEach{ $0.category = self }
That's all. You don't have to set the inverse relationship in CategoryManagedObject
To insert the data you have to decode [CategoryManagedObject]
let decoder = JSONDecoder()
decoder.userInfo[CodingUserInfoKey.context!] = dbContext
coreDataStack.deleteAllRecords("Songs")
_ = try decoder.decode([CategoryManagedObject].self, from: data)