如何计算文档数量并有效地对其进行过滤？(mongoose)

const mongoose = require('mongoose');

// 连接到MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('Connected to MongoDB');
  })
  .catch((error) => {
    console.error('Error connecting to MongoDB', error);
  });

// 定义文档模型
const mySchema = new mongoose.Schema({
  // 定义字段和类型
  name: String,
  age: Number,
  // ...
});

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

// 计算文档数量并进行过滤
MyModel.countDocuments({ age: { $gte: 18 } })
  .then((count) => {
    console.log(`There are ${count} documents with age greater than or equal to 18`);
  })
  .catch((error) => {
    console.error('Error counting documents', error);
  });