Genre detail page

種類細節頁面,需要利用_id 字段值 (自動生成) ,以呈現特定種類實例的信息。此頁面應該呈現種類名稱,各個種類的所有書本列表(每本書都連結到書本的細節頁面)。

Controller 控制器

打開 /controllers/genreController.js ,並在檔案最上方引用 asyncBook 模組。

var Book = require("../models/book");
var async = require("async");

Find the exported genre_detail() controller method and replace it with the following code.

// Display detail page for a specific Genre.
exports.genre_detail = function (req, res, next) {
      genre: function (callback) {

      genre_books: function (callback) {
        Book.find({ genre: }).exec(callback);
    function (err, results) {
      if (err) {
        return next(err);
      if (results.genre == null) {
        // No results.
        var err = new Error("Genre not found");
        err.status = 404;
        return next(err);
      // Successful, so render
      res.render("genre_detail", {
        title: "Genre Detail",
        genre: results.genre,
        genre_books: results.genre_books,

The method uses async.parallel() to query the genre name and its associated books in parallel, with the callback rendering the page when (if) both requests complete successfully.

The ID of the required genre record is encoded at the end of the URL and extracted automatically based on the route definition (/genre/:id). The ID is accessed within the controller via the request parameters: It is used in Genre.findById() to get the current genre. It is also used to get all Book objects that have the genre ID in their genre field: Book.find({ 'genre': }).

備註: If the genre does not exist in the database (i.e. it may have been deleted) then findById() will return successfully with no results. In this case we want to display a "not found" page, so we create an Error object and pass it to the next middleware function in the chain.

if (results.genre == null) {
  // No results.
  var err = new Error("Genre not found");
  err.status = 404;
  return next(err);

The message will then propagate through to our error handling code (this was set up when we generated the app skeleton - for more information see Handling Errors).

The rendered view is genre_detail and it is passed variables for the title, genre and the list of books in this genre (genre_books).


Create /views/genre_detail.pug and fill it with the text below:

extends layout

block content

  h1 Genre: #{}


    h4 Books

    each book in genre_books
        a(href=book.url) #{book.title}
      dd #{book.summary}

      p This genre has no books

The view is very similar to all our other templates. The main difference is that we don't use the title passed in for the first heading (though it is used in the underlying layout.pug template to set the page title).

What does it look like?

Run the application and open your browser to http://localhost:3000/. Select the All genres link, then select one of the genres (e.g. "Fantasy"). If everything is set up correctly, your page should look something like the following screenshot.

Genre Detail Page - Express Local Library site

備註: You might get an error similar to this:

Cast to ObjectId failed for value " 59347139895ea23f9430ecbb" at path "_id" for model "Genre"

This is a mongoose error coming from the To solve this problem, first you need to require mongoose on the genreController.js page like this:

var mongoose = require("mongoose");

Then use mongoose.Types.ObjectId() to convert the id to a that can be used. For example:

exports.genre_detail = function(req, res, next) {
    var id = mongoose.Types.ObjectId(;

Next steps