extension TestPeriods {

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

@NSManaged public var periods: [PeriodClass]?

}

public class PeriodClass: NSObject, NSCoding {


var start: Date?
var end: Date?
var period: Int16?

public func encode(with aCoder: NSCoder) {
    aCoder.encode(start, forKey: "start")
    aCoder.encode(end, forKey: "end")
    aCoder.encode(period, forKey: "period")
}


public required init?(coder aDecoder: NSCoder) {
    start = aDecoder.decodeObject(forKey: "start") as! Date
    end = aDecoder.decodeObject(forKey: "end") as! Date
    period = aDecoder.decodeObject(forKey: "period") as! Int16
}


init(start: Date, end: Date, period: Int16) {
    super.init()
    self.start = start
    self.end = end
    self.period = period
}

}

let container = TestPeriods(context: PersistenceServce.context)
// The 'period' I refer to below is just an object with the same start, end, and period and I know is not nil
let x = PeriodClass(start: period.start!, end: period.end!, period: period.period)
container.periods?.append(x)

// Saving the object here
PersistenceServce.saveContext()
// I know this Persistence Service is not a problem as I use it for other Core Data Objects

let tFetch: NSFetchRequest<TestPeriods> = TestPeriods.fetchRequest()
    do {
        let classes = try PersistenceServce.context.fetch(tFetch)
        print("TEST CLASS COUNT (classes.count)")
        ...
    }

print("TEST CLASS PERIODS (classes[0].periods)")

container.periods?.append(x)

container.periods = [x]