Mongoose - group，当集合中未找到任何内容时，计数并返回0

const mongoose = require('mongoose');

// 连接到MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

// 定义集合中的文档结构
const schema = new mongoose.Schema({
  name: String,
  age: Number
});

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

// 使用group操作进行计数并返回0
Model.aggregate([
  {
    $group: {
      _id: null,
      count: { $sum: 1 }
    }
  },
  {
    $project: {
      count: {
        $cond: {
          if: { $eq: ['$count', 0] },
          then: 0,
          else: '$count'
        }
      }
    }
  }
])
  .exec((err, result) => {
    if (err) {
      console.error(err);
    } else {
      console.log(result);
    }
  });