合并mongoose中的两个不同字段

const mongoose = require('mongoose');

// 定义模式
const schema = new mongoose.Schema({
  firstName: String,
  lastName: String
});

// 创建模型
const Model = mongoose.model('Person', schema);

// 合并字段
Model.aggregate([
  {
    $project: {
      fullName: { $concat: ['$firstName', ' ', '$lastName'] }
    }
  }
])
  .then(result => {
    console.log(result);
  })
  .catch(error => {
    console.error(error);
  });