如何从数据库（Firebase服务器端）自动删除数据并仅从firebase获取唯一查询？

Question

我想知道是否可以通过数据库规则或服务器端删除firebase数据库中的项目。现在我正在使用客户端应用程序在达到计时器后从firebase中删除值。我想避免使用客户端删除项目，因为我不希望多个用户尝试从数据库中删除项目。

其次我想知道是否有办法在firebase中进行独特的观察者查询，我只获取新数据而不是查询所有旧数据。我设置了一个地图，在添加内容时，它会查询该数据并将其添加到地图中。但它会从数据库中提供所有数据，并在我的mapview上添加多个注释。现在我可以在添加注释之前删除注释，但这非常hacky而且不是用户友好的。

代码：

class FBService {
  static let instance = FBService() 
  var firRef: FirDatabaseReference!
  private init() {
    self.fireRef = FIRDatabase.database().reference()
  }

  fun getMyAnnotations(location: CLLocation, completion: ([annotationLocation]? -> ())) {
    let now = NSDate().timeIntervalSince1970
    let locationRef = fireRef.child("annotations").queryOrderedByChild("timestamp").queryStartingAtValue(now)
    // Before I was doing fireRef.Child("annotations").queryOrderedByChild("latitude")

    locationRef.observeEventType(.Value) { (snapshot) in 
      completion(self.someMethod(snapshot))
    }
  }

  func someMethod(snapshot: FIRDataSnapshot) {}
}

class MainVC: UIViewController {
  override func viewDidLoad() {
    FBService.instance.getMyAnnotations(userLocation) { (annotations) in
      if let annotations = annotations {
      // Before I was doing mapView.removeAnnotations(mapView.annotations), which removes annotations and re-plops them as they come back.
        for anno in annotations {
          let pin = CustomAnnotation(coordinate: CLLocationCoordinate2DMake(anno.location.latitude, anno.location.longitude), name: anno.name)
          mapView.addAnnotation(anno)
        }
      }
    }
  }

}