Error Making Entity Relationship Encodable

Question

I'm trying to get a simple NSManagedObject class to conform to Encodable so that I can easily encode it to JSON.

I have an entity named TestObject that has a one to many relationship with an entity named Device.

Everything works fine until I try to encode an NSSet (to many relationship).

import UIKit
import CoreData
import Foundation

@objc(TestObject)
public class TestObject:NSManagedObject,Encodable
{

    @nonobjc public class func fetchRequest() -> NSFetchRequest<TestObject> {
        return NSFetchRequest<TestObject>(entityName: "TestObject")
    }


    @NSManaged public var testName: String?
    @NSManaged public var devices: NSSet?  // of entity type "Device"

enum CodingKeys: String, CodingKey {
        case testName
        case devices

    }
    public func encode(to encoder: Encoder) throws {
         var container = encoder.container(keyedBy:CodingKeys.self)
        try container.encode(testName,forKey:.testName)
        try container.encode(devices,forKey:.devices)
    }
}

The error I receive from the compiler is

reference to member 'devices' cannot be resolved without a contextual type
        try container.encode(devices,forKey:.devices)

What is this compiler telling me and how can I encode my objects along with its relationships?