如何获取firestore中子集合的实时更新？

Question

My database contains a subcollection called 'watched', for videos watched. I'd like to be able to keep track such subcollection in real time. The path looks like this: users/userDocument/watched/date/nestedObjectsForMoviesWatched

eg users/as7f9as98dfa/watched/June-2020/ {name: Forrest Gump, watched: true} {name: Interstellar, watched: false}

Here's how I try to handle it

  useEffect(() => {
    const unsubscribeWatched = firebase
      .firestore()
      .collection('users')
      .doc(userId)
      .collection('watched')       //why doesn't this work?
      .doc(month)
      .onSnapshot(snapshot => {
        //how to handle snapshot received?
      })

      return () => {
        unsubscribeWatched()
      }

  }, [])

It is specifically important that my app is aware of when changes are made to the watched property in a given object.

Answer 1

It seems like this documentation can help you to perform the task you want. As you can see in the shared link, "If you want to receive snapshot events when the document or query metadata changes, pass a listen options object when attaching your listener". So, it is only necessary to add "includeMetadataChanges: true" in your code, as you can see below. The complete example can be found here .

useEffect(() => {
const unsubscribeWatched = firebase
  .firestore()
  .collection('users')
  .doc(userId)
  .collection('watched') 
  .doc(month)
  .onSnapshot(snapshot => {
    includeMetadataChanges: true
  })

  return () => {
    unsubscribeWatched()
  }}