Firebase如何以原子方式防止重复条目

I'm looking at using firebase as a data store for user data for a web app. My current thought is to store each user's data using the timestamp of when they joined as the key referencing that user's data. The advantage of this scheme being that it's a simple way to assign unique integer ids to users, and makes chronological sorting of users simple.I'm looking at using firebase as a data store f