聚合时如何在Mongoose中动态设置字段？

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const mySchema = new Schema({
  // 固定字段
  name: String,
  age: Number
});

mySchema.add({ 
  // 动态字段
  dynamicField: String
});

const MyModel = mongoose.model('MyModel', mySchema);

// 创建实例并保存数据
const myInstance = new MyModel({ 
  name: 'John',
  age: 25,
  dynamicField: 'Dynamic Value'
});

myInstance.save((err, doc) => {
  if (err) {
    console.error(err);
  } else {
    console.log(doc);
  }
});