在mongoose中，如何对关联模式上的2个字段进行分组？

const mongoose = require('mongoose');

// 定义模式
const userSchema = new mongoose.Schema({
  name: String,
  age: Number
});

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  userId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'
  }
});

// 定义模型
const User = mongoose.model('User', userSchema);
const Post = mongoose.model('Post', postSchema);

// 进行关联和分组操作
Post.aggregate([
  {
    $lookup: {
      from: 'users',
      localField: 'userId',
      foreignField: '_id',
      as: 'user'
    }
  },
  {
    $group: {
      _id: '$user.name',
      totalPosts: { $sum: 1 }
    }
  }
])
.then(result => {
  console.log(result);
})
.catch(error => {
  console.error(error);
});