iOS：Firebase在启用持久性时检索旧值

Question

我正在尝试检索chatRef中最后一个孩子的时间戳。禁用持久性时，它工作正常。启用它后，它将返回一个对象而不是最后一个对象。

这怎么可能？

       self.chatRef.queryOrderedByChild("timestamp").queryLimitedToLast(1).observeSingleEventOfType(.Value, withBlock: { (snapshot) -> Void in
            for child in snapshot.children {

                let childSnapshot = snapshot.childSnapshotForPath(child.key!!)

                if let object = childSnapshot.value as? [String: AnyObject] {
                    var timestamp = (object["timestamp"] as? NSNumber)!.longLongValue 
                    timestampQueryValue = NSNumber(longLong: timestamp)
                    print("timestamp of childSnapshot: \(timestampQueryValue)") 
                    // actually prints the timestamp of an older object when persistence is enabled
                }
            }
        })
    }