使用Mongoose和Node.js将子文档保存到文档时出错

Question

首先，我想为我的英语不好而道歉。

我是Web开发主题的超级新手。目前，我正在为一个问题而苦苦挣扎。我的研究未成功，所以我希望你们中的任何人都能提供帮助。

我的问题是我无法将子文档“ post”写入我的文档“ user”中并将其保存到mongodb中。

我已经阅读了很多有关stackoverflow，mongodb，mongoose等的文档，但是我找不到适合我的解决方案。

´´´´´´

//Schema for Posts 
const postSchema = new mongoose.Schema({
  postAuthor: String,
  postTitle: String,
  postContent: String,
});

//Schema for Users 
const userSchema = new mongoose.Schema({
  username: String,
  password: String,
    posts: [postSchema]
});

//Create Collection 
const User = mongoose.model("User", userSchema);
//Create Collection 
const Post = mongoose.model("Post", postSchema);

´´´´´´´

//None working part of my code
//Save new Post as subdoc post to doc user

app.post("/compose", function(req, res) {
  User.find({
    _id: req.user._id
  }, function(err, user) {

    console.log("User found: " + user);

    const post = new Post({
      postAuthor: req.user._id,
      postTitle: req.body.postTitle,
      postContent: req.body.postContent
    });

    console.log("New Post: " + post);

    //TypeError: Cannot read property 'push' of undefined
    user.posts.push(post);
    user.save();
    console.log("Saved");

  });
});