merge-firestore.ts

private mergeAllSnapshots(data: Array<any>) {
  return combineLatest(...data)
    .switchMap(allData => {
      const combined = [];
      combined.push([].concat(...allData));
      return Observable.of(combined[0]);
    }).map(actions => {
      return actions.map(a => {
        const data = a.payload.doc.data();
        const id = a.payload.doc.id;
        return { id, ...data };
      });
  });
}