Mongoose如何在数组中查找对象

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  friends: [{
    name: String,
    age: Number
  }]
});

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

User.findOne({ 'friends.name': 'John' }, (err, user) => {
  if (err) {
    console.error(err);
  } else {
    console.log(user);
  }
});

User.find({ 'friends.age': { $gte: 18 } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.find({ friends: { $elemMatch: { name: 'John', age: { $gte: 18 } } } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});