如何用Mongoose向嵌套文档添加元素？旧代码报错求新语法

错误原因 #

你碰到的TypeError: Cannot read properties of undefined (reading 'push')错误，核心是字段名拼写错误，和Mongoose版本更新无关。你的Post模型Schema里定义的评论字段是comments（复数形式），但代码里调用的是aPost.comment（单数），这个属性不存在，自然无法调用push方法。

修正后的代码 #

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');
const Schema = mongoose.Schema;

const commentSchema = new Schema({
  text: String,
  username: String,
});

const postSchema = new Schema({
  text: String,
  username: String,
  comments: [commentSchema], // 字段为复数comments
});

const PostModel = mongoose.model('post_coll', postSchema);
const CommentModel = mongoose.model('comment_coll', commentSchema);

const aPost = new PostModel({
  text: 'one',
  username: 'two',
});

// 修正：将comment改为comments
aPost.comments.push({
  text: 'one',
  username: 'two',
});

aPost.save((err, res) => {});

额外优化建议 #

如果你的Mongoose版本是v6及以上，推荐使用async/await替代回调函数，代码可读性更强：

async function savePostWithComment() {
  try {
    await mongoose.connect('mongodb://localhost/test');
    const aPost = new PostModel({
      text: 'one',
      username: 'two',
    });
    aPost.comments.push({
      text: 'one',
      username: 'two',
    });
    const savedPost = await aPost.save();
    console.log(savedPost);
  } catch (err) {
    console.error(err);
  } finally {
    await mongoose.disconnect();
  }
}

savePostWithComment();

内容的提问来源于stack exchange，提问作者Sandra Schlichting