"data" : {
  "-LYBzlXPoN0137KRLovk" : {
    "-LYC-HHqDFgL9PovJiBr" : {
      "age" : 35,
      "country" : "Country",
      "date" : "2019-02-08T13:07:10+01:00",
      "gender" : "male",
      "id" : 1549627467620,
    },
    "age" : 35,
    "country" : "Country",
    "date" : "2019-02-08T13:04:27+01:00",
    "gender" : "male",
    "id" : 1549627467620,

exports.migrateVisits = functions.database.ref('/data/{key}/{nestedKey}')
    .onWrite((change:any, context:any) => {
        // Get a reference to the Firestore document of the changed user
        let userDoc = admin.firestore()
            .collection(`data/{key}/nestedKeys`)
            .doc(context.params.nestedKey);
        // If this user has been deleted, delete in Firestore also
        if (!change.after.exists()) {
            return userDoc.delete();
        }
        // Get the user object with the new changes,
        // as opposed to its value before the edit
        let userData = change.after.val();
        // Now update Firestore with that change

        return userDoc.set(userData);
    });