Why do I get "entity X is not key value coding-compliant"?

Question

When I run the following code:

    let saveAction = UIAlertAction(title: "Save",
                                   style: .default) {
    [unowned self] action in
    guard let textField = alert.textFields?.first,
    let mapName = textField.text else {
    return
    }
    var newCoordinate = NSEntityDescription.insertNewObject(forEntityName: "Coordinates", into: managedObjectContext)

    var newMap = NSEntityDescription.insertNewObject(forEntityName: "MapNames", into: managedObjectContext)

    newCoordinate.setValue(23, forKey: "latitude")
    newCoordinate.setValue(21, forKey: "longitude")
    newMap.setValue(mapName, forKey: "mapname")

    do
    {
    try self.appDelegate.saveContext()
    print("SAVED")
    }
    catch
    {
    }

    var request = NSFetchRequest<NSFetchRequestResult>(entityName: "Coordinates")
    request.returnsObjectsAsFaults = false

    do
    {
    var results = try managedObjectContext.fetch(request)

        if results.count > 0 {
        for result in results as! [NSManagedObject]
        {
            if let latitude = result.value(forKey: "latitude") as? Double
            {
            print(latitude)
            }
            if let longitude = result.value(forKey: "longitude") as? Double{
            print(longitude)}
            if let map = result.value(forKey: "mapname") as? String {
            print(map)}

        }
    }
    }
    catch
    {
    }

I get the following error:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ valueForUndefinedKey:]: the entity Coordinates is not key value coding-compliant for the key "mapname".'

This is weird because I do have an attribute called mapname, although it is in the MapNames entity. The error says Coordinates, why is that?

Is that because it is not possible to have 2 references to managedObjectContext in the same scope? How could I insert values to different entities then?

Answer 1

I got this same error after upgrading to Swift 4.

I had to add @objcMembers to my Entity class (or @objc to each property of the class).