Mongoose 更新 ref 和 populate ref的问题

Question

环境：

• Mongodb 3.6.3
• mongoose: 5.2.5
• nodeJS: 8.X

描述：

• 模拟博客系统，有两个collection( articles / comments), schema 如下

// article model
const ArticleSchema = new Schema({
  title: String,
  author: String,
  content: String,
  lastModified: {type: Date, default: Date.now()},
  // comments collections refs
  comments: [{type: Schema.Types.ObjectId, ref: 'comments'}],
  meta: {
    tags: {type: Array, default: []},
    votes: {type: Number, default: 0}
  }
});

// comments model
const CommentSchema = new Schema({
  // DBref to article collection
  article: {
    type: Schema.Types.ObjectId,
    ref: 'article'
  },
  // For unlogined user
  tempNick: String,
  // For future logined user
  status: Schema.Types.ObjectId,
  content: String,
  replies: [
    {
      // name for both logined & unlogined
      name: '',
      content: String,
      createdTime: {
        type: Date,
        default: Date.now()
      }
    }
  ],
  createdTime: {
    type: Date,
    default: Date.now()
  }
});

article 和 comment 是一对多的关系，因此在发布评论的方法上我是这样做的：

// Write comment
exports.postComment = function(req, res) {
  let request;
  let aid;
  console.log(chalk.green('WRITING COMMENTS'));
  request = req.body;
  request._id = new mongoose.Types.ObjectId();
  aid = req.params.aid;
  let comment = new Comment(request)
  comment.save(function(err) {
    if (err) {
      errCallback(err, res);
    } else {
      // let article = new Article({
      //   comments: comment._id
      // })
      Article.findByIdAndUpdate(aid, {$push: {comments: request._id}}, function (err, com) {
        if (err) {
          errCallback(err);
        }
        console.log('----------');
      })
      postSuccessCallback('post comment success', res);
    }
  })
}

嗯，结果是可以写入成功的，写入两次并通过mongoshell查询发现该article文档内的comments数组字段有两个内容
[ObjectId(*************),ObjectId(****************)]

但是，通过组织Mongoose来进行查询却返回undefined：

// Query for comment
// Params: { offset / limit / sortBy }
exports.getComments = function(req, res) {
  let count;
  let data;
  let { offset, limit, sortBy } = req.query;
  let { aid } = req.params

  // Get Total count
  Comment.estimatedDocumentCount(function(err, num) {
    if (err) {
      errCallback(err, res);
      return
    }
    count = num;
  });
  offset = Number(offset);
  limit = Number(limit);
  console.log('aid', aid);
  Article.findById(aid).
  populate('comments').
  // skip(offset).
  // limit(limit).
  // sort(dueSortby(sortBy)).
  exec((err, comment) => {
    console.log('execed!', comment)
    data = comment;
    getCountCallback(data, count, res);
  });
}

篇幅啰嗦，恐怕丢掉细节，如有解答望不吝告知！