无法读取 null 的属性（读取“名称”）

Question

我正在关注 Odin 的项目本地库教程（Express 应用程序）。 这是我的代码（仅相关部分）：

在routes/catalog.js我有这个

router.get("/books", book_controller.book_list);

模型/book.js

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const BookSchema = new Schema({
    title:{type:String,required:true},
    author:{type:Schema.Types.ObjectId,ref:"Author",required:true},
    summary:{type:String,required:true},
    isbn:{type:String,required:true},
    genre:[{type: Schema.Types.ObjectId,ref:"Genre"}],
});
BookSchema.virtual("url").get(function(){
    return `/catalog/book/${this._id}`
})

module.exports = mongoose.model("Book",BookSchema);

控制器/bookController.js

const Book = require("../models/book");
const Author = require("../models/author");
const Genre = require("../models/genre");
const BookInstance = require("../models/bookinstance");

const asyncHandler = require("express-async-handler");

exports.index = asyncHandler(async (req, res, next) => {
  // Get details of books, book instances, authors and genre counts (in parallel)
  const [
    numBooks,
    numBookInstances,
    numAvailableBookInstances,
    numAuthors,
    numGenres,
  ] = await Promise.all([
    Book.countDocuments({}).exec(),
    BookInstance.countDocuments({}).exec(),
    BookInstance.countDocuments({ status: "Available" }).exec(),
    Author.countDocuments({}).exec(),
    Genre.countDocuments({}).exec(),
  ]);

  res.render("index", {
    title: "Local Library Home",
    book_count: numBooks,
    book_instance_count: numBookInstances,
    book_instance_available_count: numAvailableBookInstances,
    author_count: numAuthors,
    genre_count: numGenres,
  });
});

exports.book_list = asyncHandler(async (req, res, next) => {
  const allBooks = await Book.find({}, "title author")
    .sort({ title: 1 })
    .populate("author")
    .exec();

  res.render("book_list", { title: "Book List", book_list: allBooks });
});

现在这是导致问题的文件：views/book_list.pug

extends layout

block content
  h1= title

  ul
    each book in book_list
      li
        a(href=book.url) #{book.title}
        |  (#{book.author.name})

    else
      li There are no books.

我不知道为什么会导致错误？这显然应该有效，并且在教程中也有效，所以我这里一定有问题，但看在上帝的份上，我找不到错误。 当我转到路线 /catalog/books 时出现错误

D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\views\book_list.pug:10 8| li 9| a(href=book.url) #{book.title} > 10| | (#{book.author.name}) 11| 12| else 13| li There are no books. Cannot read properties of null (reading 'name')
TypeError: D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\views\book_list.pug:10
    8|       li
    9|         a(href=book.url) #{book.title}
  > 10|         |  (#{book.author.name})
    11| 
    12|     else
    13|       li There are no books.

Cannot read properties of null (reading 'name')
    at eval (eval at wrap (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug-runtime\wrap.js:6:10), <anonymous>:120:69)
    at eval (eval at wrap (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug-runtime\wrap.js:6:10), <anonymous>:155:4)
    at template (eval at wrap (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug-runtime\wrap.js:6:10), <anonymous>:157:155)
    at exports.renderFile (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug\lib\index.js:418:38)
    at exports.renderFile (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug\lib\index.js:408:21)
    at exports.__express [as engine] (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\pug\lib\index.js:455:11)
    at View.render (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\express\lib\view.js:135:8)
    at tryRender (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\express\lib\application.js:640:10)
    at Function.render (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\express\lib\application.js:592:3)
    at ServerResponse.render (D:\THE ODIN PROJECT\NodeJSCourse\express-locallib-pug\node_modules\express\lib\response.js:1008:7)

我一个人去，只是看看总体思路。当我第一次遇到这个问题时，我认为这是因为我使用的是 EJS，并且教程是由 Pug 进行的。于是我赶紧将文件和视图引擎切换为pug，但根本没有帮助。然后我认为这可能是兼容性问题，所以我通过本地生成项目作为 pug 并复制其余文件来完全克隆这个项目。错误仍然存在——仍然同时存在。 当我删除 book_list.pug 时，没有错误（显然这是由

引起的）

 > 10|         |  (#{book.author.name})

line，但是我不知道为什么这会导致我的问题，并且在教程中有效？我也找不到解决方案。

Answer 1

检查您的 项目中的作者模型 是否包含Name字段 或者名称可能为空，请检查 mongodb 检查作者模式中是否存在数据