如何将评论的主题自动设置为与帖子相同的主题

const express = require('express');
const mongoose = require('mongoose');

const app = express();
app.use(express.json());

mongoose.connect('mongodb://localhost:27017/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  // 其他字段
});

const commentSchema = new mongoose.Schema({
  postId: { type: mongoose.Schema.Types.ObjectId, ref: 'Post' },
  content: String,
  topic: String,
  // 其他字段
});

const Post = mongoose.model('Post', postSchema);
const Comment = mongoose.model('Comment', commentSchema);

app.post('/comments', async (req, res) => {
  const { postId, content } = req.body;
  const post = await Post.findById(postId);
  if (!post) {
    return res.status(404).send('Post not found');
  }
  const comment = new Comment({
    postId,
    content,
    topic: post.title // 设置评论主题为帖子标题
  });
  await comment.save();
  res.send(comment);
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

import React, { useState, useEffect } from 'react';
import axios from 'axios';

function CommentForm({ postId }) {
  const [content, setContent] = useState('');
  const [topic, setTopic] = useState('');

  const handleSubmit = async (e) => {
    e.preventDefault();
    const response = await axios.post('/comments', { postId, content });
    setTopic(response.data.topic); // 设置评论主题
    setContent('');
  };

  return (
    <form onSubmit={handleSubmit}>
      <input type="text" value={content} onChange={(e) => setContent(e.target.value)} />
      <button type="submit">Submit</button>
    </form>
  );
}

function Comment({ comment }) {
  return (
    <div>
      <h3>{comment.topic}</h3>
      <p>{comment.content}</p>
    </div>
  );
}

function App() {
  const [comments, setComments] = useState([]);

  useEffect(() => {
    const fetchComments = async () => {
      const response = await axios.get('/comments');
      setComments(response.data);
    };
    fetchComments();
  }, []);

  return (
    <div>
      {comments.map(comment => (
        <Comment key={comment._id} comment={comment} />
      ))}
      <CommentForm postId="somePostId" />
    </div>
  );
}

export default App;