Core Data: Cannot save array with NSObjects to Transformable entity

Question

I would like to store an array which includes NSObjects into a core data entity. I did several things trying to run it correctly but everytime I change something I get other issues.

This is my Entity named P_TrainingPlans

I can store the name and date attribute without any problems.

let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext
    let trainPlan = P_TrainingPlan(context : context)

    createTrainingPath() //adds objects of type TrainingTool to selectedTrainingTools array
    trainPlan.date = NSDate.init()
    trainPlan.name = nameOfPlan.text

    //trainPlan.trainingPath = selectedTrainingTools as NSArray?

    appDelegate.saveContext()

The out-commented code doesnt work - thats the problem..

My aim is to store NSObjects of type TrainingTool into the trainingPath attribute of type Transformable.

How can I do that?

I also tried to create another entity for TrainingTool instead of a TrainingTool as NSObject class. Then the save process worked but the app crashed when fetching P_TrainingPlan entity..