Node.js 使用NodeJS中的promiss从两个不同的SQL查询向EJS视图传递两个对象

我正在尝试构建一个包含三个字段的EJS表单，我需要同时向它传递两组数据：用户和使用承诺的书籍。不幸的是，书籍并没有通过考试，而且“未定义”。我不明白为什么

表格

Textfield（与本例无关）

带有用户列表的下拉框

带有书籍列表的下拉框

对于（2）和（3），我查询我的mysql数据库以获取数据，以便填写表单下拉框

/controller.js

const User       = require('../models/user.js');
const Book       = require('../models/book.js');

    exports.getAddNeueAusleihe = (req, res, next) => {

      // storing users and books in this object
      let view_data_for_my_view = {};

      // fetch users for dropdown-box nr.1
      User.fetchAll()

      .then(([users_rows]) => {
        view_data.users = users_rows;
        
        // fetch books for dropdown-box nr. 2
        return Book.fetchAll(([books_rows]) => {
        view_data.books = books_rows;
       });
      })

      .then(() => {
        
        // send data to view
        res.render('neue-ausleihe.ejs', {

          users: view_data.users,
          books: view_data.books,
          pageTitle:  'Neue Ausleihe'

        });
      });
    }

const db = require('../util/database.js');

module.exports = class User {
    constructor(id, name) {
      this.id = id;
      this.name = name;
    }

    static fetchAll() {
      return db.execute('SELECT * from dbuser WHERE UserFlag = "active"');
   };
}

const db = require('../util/database.js');    

module.exports = class Book {
        constructor(id, name) {
          this.id = id;
          this.name = name;
        }
    
        static fetchAll() {
          return db.execute('SELECT * from books WHERE status = "free"');
       }
    }

用户获取工作正常。但是Book fetch确实返回“undefined”，尽管books模型中的SQL代码工作正常。它实际上跳转到books模型中，但不会将值添加到视图中。这是我的模型SQL代码

/models/user.js

const User       = require('../models/user.js');
const Book       = require('../models/book.js');

    exports.getAddNeueAusleihe = (req, res, next) => {

      // storing users and books in this object
      let view_data_for_my_view = {};

      // fetch users for dropdown-box nr.1
      User.fetchAll()

      .then(([users_rows]) => {
        view_data.users = users_rows;
        
        // fetch books for dropdown-box nr. 2
        return Book.fetchAll(([books_rows]) => {
        view_data.books = books_rows;
       });
      })

      .then(() => {
        
        // send data to view
        res.render('neue-ausleihe.ejs', {

          users: view_data.users,
          books: view_data.books,
          pageTitle:  'Neue Ausleihe'

        });
      });
    }

const db = require('../util/database.js');

module.exports = class User {
    constructor(id, name) {
      this.id = id;
      this.name = name;
    }

    static fetchAll() {
      return db.execute('SELECT * from dbuser WHERE UserFlag = "active"');
   };
}

const db = require('../util/database.js');    

module.exports = class Book {
        constructor(id, name) {
          this.id = id;
          this.name = name;
        }
    
        static fetchAll() {
          return db.execute('SELECT * from books WHERE status = "free"');
       }
    }

/models/books.js

const User       = require('../models/user.js');
const Book       = require('../models/book.js');

    exports.getAddNeueAusleihe = (req, res, next) => {

      // storing users and books in this object
      let view_data_for_my_view = {};

      // fetch users for dropdown-box nr.1
      User.fetchAll()

      .then(([users_rows]) => {
        view_data.users = users_rows;
        
        // fetch books for dropdown-box nr. 2
        return Book.fetchAll(([books_rows]) => {
        view_data.books = books_rows;
       });
      })

      .then(() => {
        
        // send data to view
        res.render('neue-ausleihe.ejs', {

          users: view_data.users,
          books: view_data.books,
          pageTitle:  'Neue Ausleihe'

        });
      });
    }

const db = require('../util/database.js');

module.exports = class User {
    constructor(id, name) {
      this.id = id;
      this.name = name;
    }

    static fetchAll() {
      return db.execute('SELECT * from dbuser WHERE UserFlag = "active"');
   };
}

const db = require('../util/database.js');    

module.exports = class Book {
        constructor(id, name) {
          this.id = id;
          this.name = name;
        }
    
        static fetchAll() {
          return db.execute('SELECT * from books WHERE status = "free"');
       }
    }

---

假设

db.execute

返回解析为数组的承诺，其中第一个条目是实际结果，那么您的代码应该如下所示：

exports.getAddNeueAusleihe = (req, res, next) => {

  // storing users and books in this object
  const view_data = {};

  // fetch users for dropdown-box nr.1
  User.fetchAll()
   .then(([users_rows]) => {
      view_data.users = users_rows;
    
      // fetch books for dropdown-box nr. 2
      return Book.fetchAll();
    })
    .then([book_rows] => {
      view_data.books = books_rows;
    })
    .then(() => {
    
      // send data to view
      res.render('neue-ausleihe.ejs', {
        users: view_data.users,
        books: view_data.books,
        pageTitle:  'Neue Ausleihe'
      });
    });
}

奖励版本：

exports.getAddNeueAusleihe = async(req, res, next) => 
    
  // send data to view
  res.render('neue-ausleihe.ejs', {
     pageTitle:  'Neue Ausleihe',
     users: (await User.fetchAll())[0]
     books: (await Book.fetchAll())[0]
  });

}

---

即使是写这篇文章的人，我仍然不知道基于异步/等待的代码与承诺链相比可读性有多高。

db.execute返回什么？顺便说一句，您不必在javascript中使用

static

。它不是Java。可以在类之外创建函数。奇怪的是，我在任何地方都看不到

新书

，因为它的意图看起来像是代表书籍实体。这两个'db.execute'都返回正确的结果。对于用户来说，它基本上是“（用户id、用户名、userflag）”。对于书籍，它是“（书籍id、书籍名称、状态）”。还有一些属性，但它们对这个例子来说毫无意义。耶！成功了。我想知道，是否有一种方法可以解决这个问题，而不必构造“view\u data”对象？因为view_数据实际上变成了一个复合对象，它承载两种不同的数据类别——用户和书籍。是的，使用async您当然可以。我将更新“奖金版本”