ios - Realm: Can't update a object if one of its properties is another object that already exists -

i have schema:

class a: object {     var ida = ""     var b: b?     override class func primarykey() -> string {         return "ida"     } }  class b: object {    var idb = ""    var name = ""    override class func primarykey() -> string {         return "idb"     } } 

so if want save object:

func updatea(a: a, b: b) {  {    try realm.write {        a.b = b  //here excepcion        realm.add(a, update: true)    }  } catch { error   }  } 

when call update can't assign b a.b, get: *** terminating app due uncaught exception 'rlmexception', reason: 'can't create object existing primary key value "bkey".' if b object exists in database primary key that, if new object works. im pretty sure long time ago worked expected notice want save updating b object if 1 of properties has changed, name. , dont wanto create b object, use b object passed.

it's expected isnt it, adding b created create similar one, that's not possible since having b primary key already

what should create a new b, not create b first assign a