如何在firebase flutter中合并两个集合？

import 'package:cloud_firestore/cloud_firestore.dart';
final firestore = FirebaseFirestore.instance;

void mergeCollections() async {
  final QuerySnapshot snapshot1 = await firestore.collection('collection1').get();
  final QuerySnapshot snapshot2 = await firestore.collection('collection2').get();

  final List<QueryDocumentSnapshot> documents1 = snapshot1.docs;
  final List<QueryDocumentSnapshot> documents2 = snapshot2.docs;

  documents1.forEach((document) async {
    // 获取文档数据
    final data = document.data();

    // 获取文档ID
    final documentId = document.id;

    // 将文档数据存入第二个集合
    await firestore.collection('collection2').doc(documentId).set(data);
  });
}