Swift: Query one last entry from Firebase, with completion handler

firebase-query.swift

func getTheLastGuestKey(completionHandler: (Bool)-> ()) {
        
        self.guestRef.queryOrderedByKey().queryLimitedToLast(1).observeSingleEventOfType(.Value, withBlock: {(snapshot) in
            for guest in snapshot.children {
                let guestSnapshot = snapshot.childSnapshotForPath(guest.key)
                var lastGuestCheckintime: String! = ""
                
                if let result_checkin = guestSnapshot.value?["checkintime"] as? NSNumber {
                    lastGuestCheckintime = "\(result_checkin)"
                }
                //let lastGuestCheckintime = guestSnapshot.value?["checkintime"] as! String
                
                if (lastGuestCheckintime == "") {
                    self.shouldIncrementSequence = false
                    self.currentGuestKey = guest.key
                    print("last guest checkintime is blank")
                }
                else {
                    self.shouldIncrementSequence = true
                    print("last guest checkintime is NOT blank")
                }
            }
            
            completionHandler(true)
        }) { (error) in
            print(error.localizedDescription)
            completionHandler(false)
        }
    }