Firebase 实时数据库访问来自已删除节点的数据数组

Question

I am deleting a FRTDB node, I want to access deleted data from that node. the functions looks as follow:

exports.events = functions.database.ref('/events/{eventId}').onWrite(async (change, context) => {
  const eventId = context.params.eventId
  if (!change.after.exists() && change.before.exists()) {
    //data removed
    return Promise.all([admin.database().ref(`/events/${eventId}/dayofweek`).once('value')]).then(n => {
      const pms = []
      const days = n[0]
      days.forEach(x => {
        pms.push(admin.database().ref(`${change.before.val().active ? 'active' : 'inactive'}/${x.key}/${eventId}`).set(null))
      })
      return Promise.all(pms)
    });
   else {
    return null;
  }
})

The probem I am having is that

admin.database().ref(`/events/${eventId}/dayofweek

do not loop the data because it seems data is no longer there so the forEach is not working. How can I get access to this data and get to loop the deleted data?

Answer 1

Of course you won't be able to read data that was just deleted. The function runs after the delete is complete. If you want to get the data that was just deleted, you're supposed to use change.before as described in the documentation :

The Change object has a before property that lets you inspect what was saved to Realtime Database before the event. The before property returns a DataSnapshot where all methods (for example, val() and exists()) refer to the previous value. You can read the new value again by either using the original DataSnapshot or reading the after property. This property on any Change is another DataSnapshot representing the state of the data after the event happened.

Answer 2

The data that was deleted from the database is actually included in the call to your Cloud Function. You can get if from change.before .

exports.events = functions.database.ref('/events/{eventId}').onWrite(async (change, context) => {
  const eventId = context.params.eventId
  if (!change.after.exists() && change.before.exists()) {
    //data removed
    days = change.before.val().dayofweek;
    ...
})