如何使用Node.js、Express和Mongoose创建多个API

我正在try 使用模式模式在node.js中创建多个Express API.我面临的问题是，如果我创建两个以上的API，那么第三个(最后一个)就不起作用了.

正如您在下面的屏幕截图中所看到的.3号.

展示所有产品.

显示单一产品.

未显示所有类别.

1.showing entire collection.

2.showing single row of collection based on ID.

3.showing collection of specific column.

here is module schema:

const mongoose = require('mongoose');

const ProductSchema = new mongoose.Schema({
    title: String,
    description: String,
    category: String,
})

const PostModel = mongoose.model("products", ProductSchema) //"table name", UserSchema
module.exports = PostModel

here is index.js

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

//export module schema
const PostModel = require('./models/CreatePost')

// Middleware
const app = express()
app.use(cors());
app.use(express.json());

//define port
const port = 5000; 

//define database
mongoose.connect("mongodb+srv://username:<password>@cluster0.jvxew8i.mongodb.net/store")



//find by table ID
app.get('/products/:_id', function(req, res) {
  PostModel.findById(req.params._id)
  .then(products => res.json(products))
  .catch(err => res.json(err))
})


//find by table
app.get('/products', function(req, res) {
  PostModel.find()
  .then(products => res.json(products))
  .catch(err => res.json(err))
})


//find by specific column
app.get('/products/categories', function(req, res) {
  PostModel.find({}, {category: 1, _id: 0})
  .then(products => res.json(products))
  .catch(err => res.json(err))
})


// Server listen
app.listen(port, () => {
    console.log(`Server is running on port: ${port}`);
})