ios - updating with Core Data -

i have following code:

class ratingsdata {      var ratings = [nsmanagedobject]()      func fetchfromdatabase() -> void {         let appdelegate = uiapplication.sharedapplication().delegate as! appdelegate         let managedcontext = appdelegate.managedobjectcontext!         let fetchrequest = nsfetchrequest(entityname: "rating")         var error: nserror?          let sortdescriptor = nssortdescriptor(key: "date", ascending: false)         fetchrequest.sortdescriptors = [sortdescriptor]          let fetchedresults = managedcontext.executefetchrequest(fetchrequest, error: &error) as! [nsmanagedobject]?          if let results = fetchedresults {             ratings = results         } else {             println("error fetching database")         }      }      func addtitle(newtitle:string) -> void {          let appdelegate = uiapplication.sharedapplication().delegate as! appdelegate         let managedcontext = appdelegate.managedobjectcontext!         let entitydescription = nsentitydescription.entityforname("rating", inmanagedobjectcontext: managedcontext)          let title = rating(entity: entitydescription!, insertintomanagedobjectcontext: managedcontext)          //new way         title.title = newtitle         println("new rating title \(newtitle) saved")          var error: nserror?         if !managedcontext.save(&error) {             println("could not save")         }      }      func addstar(title: string, star: nsdata) -> void {          let appdelegate = uiapplication.sharedapplication().delegate as! appdelegate         let managedcontext = appdelegate.managedobjectcontext!         let entitydescription = nsentitydescription.entityforname("rating", inmanagedobjectcontext: managedcontext)          let request = nsfetchrequest()         request.entity = entitydescription          let predicate = nspredicate(format: "title  = %@", title)         request.predicate = predicate          var error: nserror?         var results = managedcontext.executefetchrequest(request, error: &error) as! [nsmanagedobject]?          results?[0].setvalue(star, forkey: "star")         println("new rating star saved")          if !managedcontext.save(&error) {             println("could not save")         }      } }     

rating entity, title , star attribute. works fine, when tap onto same title change rating, app crashes. when tapping on same title, able update star image , display in table view.

i can't figure out how it.