return this.afs.collection("events").snapshotChanges().pipe(
      map(eventSnapshots => {
        return eventSnapshots.reduce((eventsArray: EventInterface[], eventSnapshot) => {
          eventsArray.push(EventImporterJSON.getEventFromJSON(<EventJSONInterface>eventSnapshot.payload.doc.data()));
          return eventsArray;
        }, []);
      }),
    );

return this.afs.collection("events").snapshotChanges().pipe(
          map(eventSnapshots => {
            return eventSnapshots.reduce((eventsArray: EventInterface[], eventSnapshot) => {
              eventsArray.push(EventImporterJSON.getEventFromJSON(<EventJSONInterface>eventSnapshot.payload.doc.data()));
        // Here I need the help the below line returns an Observable<Snapshots[]>
this.afs.collection("events").doc(eventSnapshot.payload.doc.id).collection('activities').snapshotchanges()

              return eventsArray;
            }, []);
          }),
        );