Rocket.Chat/app/lib/server/functions/updateGroupDMsName.js

import { Rooms, Subscriptions, Users } from '../../../models/server';

const getFname = (members) => members.map(({ name, username }) => name || username).join(', ');
const getName = (members) => members.map(({ username }) => username).join(',');

function getUsersWhoAreInTheSameGroupDMsAs(user) {
	// add all users to single array so we can fetch details from them all at once
	const rooms = Rooms.findGroupDMsByUids(user._id, { fields: { uids: 1 } });
	if (rooms.count() === 0) {
		return;
	}

	const userIds = new Set();
	const users = new Map();

	rooms.forEach((room) => room.uids.forEach((uid) => uid !== user._id && userIds.add(uid)));

	Users.findByIds([...userIds], { fields: { username: 1, name: 1 } }).forEach((user) => users.set(user._id, user));

	return users;
}

function sortUsersAlphabetically(u1, u2) {
	return (u1.name || u1.username).localeCompare(u2.name || u2.username);
}

export const updateGroupDMsName = (userThatChangedName) => {
	if (!userThatChangedName.username) {
		return;
	}

	const users = getUsersWhoAreInTheSameGroupDMsAs(userThatChangedName);
	if (!users) {
		return;
	}

	users.set(userThatChangedName._id, userThatChangedName);

	const rooms = Rooms.findGroupDMsByUids(userThatChangedName._id, { fields: { uids: 1 } });

	const getMembers = (uids) => uids.map((uid) => users.get(uid)).filter(Boolean);

	// loop rooms to update the subcriptions from them all
	rooms.forEach((room) => {
		const members = getMembers(room.uids);
		const sortedMembers = members.sort(sortUsersAlphabetically);

		const subs = Subscriptions.findByRoomId(room._id, { fields: { '_id': 1, 'u._id': 1 } });
		subs.forEach((sub) => {
			const otherMembers = sortedMembers.filter(({ _id }) => _id !== sub.u._id);
			Subscriptions.updateNameAndFnameById(sub._id, getName(otherMembers), getFname(otherMembers));
		});
	});
};