如何在mongoose中查找另一个集合中的集合

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

const PostSchema = new mongoose.Schema({
  title: String,
  content: String
});

const User = mongoose.model('User', UserSchema);
const Post = mongoose.model('Post', PostSchema);

User.findOne({ name: 'John' })
  .populate('posts')
  .exec(function(err, user) {
    if (err) throw err;
    console.log(user);
  });

User.aggregate([
  {
    $lookup: {
      from: 'posts',
      localField: 'posts',
      foreignField: '_id',
      as: 'postsData'
    }
  }
])
.exec(function(err, users) {
  if (err) throw err;
  console.log(users);
});

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

const PostSchema = new mongoose.Schema({
  title: String,
  content: String
});

const User = mongoose.model('User', UserSchema);
const Post = mongoose.model('Post', PostSchema);

User.findOne({ name: 'John' })
  .populate('posts')
  .exec(function(err, user) {
    if (err) throw err;
    console.log(user);
  });