无法查询子单据mongoose

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('Connected to MongoDB');
  })
  .catch((error) => {
    console.error('Failed to connect to MongoDB', error);
  });

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

// 定义子单据模型
const SubDocumentSchema = new Schema({
  // 子单据的属性
  // ...
});

// 定义主单据模型
const MainDocumentSchema = new Schema({
  // 主单据的属性
  // ...
  subDocuments: [SubDocumentSchema] // 子单据的引用
});

// 导出模型
const MainDocument = mongoose.model('MainDocument', MainDocumentSchema);

// 查询主单据及其子单据
MainDocument.findOne({ _id: '123' })
  .populate('subDocuments') // 关联查询子单据
  .exec((error, mainDocument) => {
    if (error) {
      console.error('Failed to query main document', error);
    } else {
      console.log(mainDocument);
    }
  });