您应该使用firebase云消息传递，而不是在cloudfirestore中搜索要通知的用户。这允许您为每个用户订阅与其所在组相对应的主题。
因为您需要为每个相关用户订阅相应的通知主题，所以这不是一个简单的“drop-in and-it-works”解决方案。关于如何做到这一点，请参阅目标平台的文档。

import * as functions from "firebase-functions";
import * as admin from "firebase-admin";

export const notifyGroupsOfNewPost = functions.database.ref("/Poster/{postId}").onCreate(async (snapshot, context) => {

  /* avoid using `snapshot.val()` unless you need all of the data at once */

  const postGroups = snapshot.child("Groups").val(); // assumed to be parsed as an array, make sure to handle non-array values gracefully

  if (postGroups === null) {
    throw new Error('Groups of new post missing');
  }

  return Promise.all(postGroups.map((group) => {
    const message = {
      /* note: only send data used for creating the notification message,
         the rest of it can be downloaded using the SDK once it's needed */
      data: { 
        Impact: snapshot.child("Impact").val(),
        Subject: snapshot.child("Subject").val(),
        TT: snapshot.child("TT").val()
      },
      topic: group /* you could also use `${group}-newpost` here */
    };

    return admin.messaging().send(message)
      .then((messageId) => {
        console.log(`Successfully notified the ${group} group about new post #${snapshot.key} (FCM #{$messageId})`);
      })
      .catch((error) => {
        console.error(`Failed to notify the ${group} group about new post #${snapshot.key}:`, error);
      });
  });
});