Mongoose获取从一个集合到另一个集合的关系的大小

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  posts: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Post' }]
});

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});

const User = mongoose.model('User', userSchema);
const Post = mongoose.model('Post', postSchema);

User.findById(userId)
  .populate('posts')
  .exec((err, user) => {
    if (err) {
      console.error(err);
    } else {
      const postCount = user.posts.length;
      console.log(`User ${user.name} has ${postCount} posts.`);
    }
  });