how to save Related user in a object

i’m trying to create an object in my table say CAR , inside my table i have a column driver which is related to Users (one to one relation) and when the user creates a car object i want to put the current user in driver column for that i’m trying this :
func createCar(carString :String ){

currentUser = backendless.userService.currentUser

let car = Cars() = self.ID = carString

let user = Users() // this is my Users class
user.loggedUser = currentUser // here loggedUser is of BackendlessUser Type

print(“currentUser : (currentUser)”)

car.driver? = user

response: { (result: AnyObject!) -> Void in
let obj = result as! Cars


error: { (fault: Fault!) -> Void in

print(“fServer reported an error: (fault)”)

but still my driver column is empty ?? am i doing something wrong ?

You need to relate the Car object to BackendlessUser, not to your custom Users object.