如何使用ReactJS获取MongoDB值并显示在表中

Question

I am trying to populate data from my MongoDB database in a table using ReactJS . But in return I get nothing but a html page in return and these are are shown in the browser console.

And this is how my React looks like to get the data using axios

import React, { Component } from "react";
import "react-datepicker/dist/react-datepicker.css";
import axio from "axios";

const ShowAssignments = (props) => (
  <tr>
    <td>{props.assignment.assignmentName}</td>
    <td>{props.assignment.assignmentDescription}</td>
    <td>{props.assignment.courseName}</td>
    <td>{props.assignment.assignmentDueDate}</td>
  </tr>
);

export default class AllAssignments extends Component {
  constructor(props) {
    super(props);

    this.state = {
      assignmentName: "",
      assignmentDescription: "",
      courseName: "",
      assignmentDueDate: "",
      allAssignments: []
    };
  }

  componentDidMount() {
    axio
      .get("http://localhost:4000/courseweb/assignments")
      .then(res => {
        this.setState({
          allAssignments: res.data
        });
      })
      .catch(err => {
        console.log(err);
      });
  }

  getRows() {
    return this.state.allAssignments.map((currentAssignment, id) => {
        return <ShowAssignments book={currentAssignment} key={id} />
    });
  }
  render() {
    return (
      <div id="content-wrapper">
        <div className="container-fluid">
          <ol className="breadcrumb">
            <li className="breadcrumb-item">
              <a href={`/instructor/${this.props.username}`}>Home</a>
            </li>
            <li className="breadcrumb-item active">Update Assignment</li>
          </ol>

          <h1>Update Assignments</h1>
          <hr />
          <p>Update due dates of Assignments</p>
          <br />
          <table className="table table-striped">
            <thead>
              <tr>
                <th>Assignment Name</th>
                <th>Assignment Description</th>
                <th>Course Name</th>
                <th>Due Date</th>
                <th>Action</th>
              </tr>
            </thead>
            <tbody>{this.getRows()}</tbody>
          </table>
        </div>
      </div>
    );
  }
}

And this is my server.js which handles the backend.

const Bundler = require("parcel-bundler");


const bundler = new Bundler("./src/index.html");

const express = require("express");
const bodyParser = require("body-parser");
const cors = require("cors");
const mongoose = require("mongoose");

const InstructorDB = require("./public/DBModels/InstructorDB");
const AssignmentDB = require('./public/DBModels/AssignmentDb');

const app = express();
const router = express.Router();

app.use(bodyParser.json());
app.use(cors());
app.use("/courseweb", router);
app.use(bundler.middleware());

//connect to books database
mongoose.connect("mongodb://127.0.0.1:27017/courseweb", { useNewUrlParser: true });

const connection = mongoose.connection;
connection.once("open", () => {
  console.log("Connected to MongoDB via port 27017");
});

app.listen(4000, () => {
  console.log("Listening to port 4000");
});

//add a course - not my part though
router.route("/course/add").post((req, res) => {
  let instructorDB = new instructorDB(req.body);
  instructorDB
    .save()
    .then(bookDB => {
      res.status(200).send(`${bookDB} Added!`);
    })
    .catch(err => {
      res.status(400).send({ message: err });
    });
});

//get courses to-be accepted count
router.route("/courses").get((req, res) => {
  InstructorDB.countDocuments({}, function(err, count) {
    res.status(200).send(`${count}`);
  });
});

//add an assignment
router.route('/assignment/add').post((req, res) => {
  let assignmentDb = new AssignmentDB(req.body);
  assignmentDb.save().then((assignment) => {
    res.status(200).send(assignment);
  }).catch((err) => {
    res.status(400).send({message: err});
  });
});

//get all assignments, this is where I should get the values for this question
router.route('/assignments').get((req, res) => {
  AssignmentDB.find((err, assignments) => {
    if(err) throw err;
    res.status(200).send("Hi");
  });
});

Can someone please tell me where I have gone wrong?

Answer 1

Make it defensive

First step in making more solid code is to have an empty state or some sort of protection against crashes.

A simple fix here that will show you more info is this line

  getRows() {
    if (!this.state.allAssignments && !this.state.allAssignments.length) return null;
    return this.state.allAssignments.map((currentAssignment, id) => {
        return <ShowAssignments book={currentAssignment} key={id} />
    });
  }

then make some logs to see what values you have in the state at each rerender.

  getRows() {
    console.log(this.state);
    if (!this.state.allAssignments && !this.state.allAssignments.length) return null;
    return this.state.allAssignments.map((currentAssignment, id) => {
        return <ShowAssignments book={currentAssignment} key={id} />
    });
  }

Do this first and see if there's even data in your state. It will probably show that you need to implement the lifecycle method componentDidUpdate to handle data coming in.