Node.js 如何搜索mongoose模式对象？

我正在尝试搜索所有注释，并返回特定用户从我的mongodb创建的所有注释，但当我尝试搜索时，返回的数组为空

我试过：

Comment.find({author: {$elemMatch: {username: req.params.username}}}, (err, foundComments) => {
        // Code goes here
      });

Mongoose注释模式：

const mongoose = require('mongoose');

const commentSchema = new mongoose.Schema({
  text: String,
  discussion:[{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Comment',
  }],
  createdAt: {
    type: Date,
    default: Date.now,
  },
  author: {
    id: {
      type: mongoose.Schema.Types.ObjectId,
      ref: 'User',
    },
    username: 'String',
    avatar: {
      image: String,
      imageId: String,
    },
  },
  likes: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User',
  }],
  dislikes: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User',
  }],
});

module.exports = mongoose.model('Comment', commentSchema);

当我运行它时，我希望

foundComments

是一个数组

[array of comments]

，但我只得到一个空数组

[]

Comment.find（{“author.username”：req.params.username}）注释.find（{“author.username”：req.params.username}）

Comment.find({"username": req.params.username},{"discussion":1,"_id":0})