从数据库返回数据到控制器

Question

I am building a new API in Express using an MVC architecture and I cannot get the data to return to the controllers file from the db access file.

I started this out trying to use callback functions and then switched to promises, but i'm pretty sure I am missing something about the scopes that is causing my problem. I have racked my brain as well as Google over and over and worked on this for a couple days to no avail.

//This is the controllers file code

let bodyparser = require('body-parser');
const dbUtilities = require('../db/db.js');

let controller = {
  retrieveTables: function(req, res){
    database = req.params.database;
    query = ["SHOW TABLES FROM " +database +";"];
    connectDb(query, database, function(results){
      res.render('dashboard', {data: results});
      console.log('results from controller' + results);
    });
  },

//This is my db.js file that handles data retrieval from mysql

var mysql = require('mysql');

//generalized utility db functions
//function to open a db connection

//opening the connection to the database
connectDb = function(query, database){  
resultData = [];
  var connectionPromise = dbConnect(database);
    connectionPromise.then(function(connection){
      for(var i = 0; i < query.length; i++){
        var queryPromise = buildQuery(connection, query[i]);
        queryPromise.then(function(result){
          console.log('QUERY' + i + ': ' + query[i]);
          console.log(result);
          resultData.push(result);
          if(resultData.length == query.length){
            dbDisconnect(connection);
            return(resultData);
          }
        })
      }
    });

//opening the db connection
function dbConnect(database){
  return new Promise(function(resolve, reject){
    var connection = mysql.createConnection({
      host : 'localhost',
      user : 'rcg',
      password: '8693ViaMallorca',
      database: database
    });

    connection.connect(function(err) {
      if(err) {
        console.error('error connecting: ' + err.stack);
        reject(error);
      }
      else{
        console.log('connected as id ' + connection.threadID);
        resolve(connection);
      }
    });
  });
}

//building the query promise function
function buildQuery(connection, query){
  return new Promise(function(resolve, reject){
    connection.query(query, function(error, results, fields){
      if(error){
        console.error('error retrieving the query');
        console.log(query);
        reject(error);
      }
      else{
        resolve(results);
      };
    });
  });
}

//closing the connection to the database
function dbDisconnect(connection){
  connection.end(function(err) {
    if(err) {
      console.error('error terminating connection');
      return(error);
    }
    else{
      return(console.log('connection successfully terminated'));
    }
  });
}

}
module.exports = connectDb;

Answer 1

The reason why this wasn't working was because you were returning only in the inner callback. You needed to return in outer callback too. The data had no return path to get back to the callee (eg: the controller). Since you mentioned async/await, I re-wrote this to reflect modern async/await syntax.

const connectDb = async function(query, database){  
  resultArray = [];
  const connection = await dbConnect(database);

  for(let i = 0; i < query.length; i++) {
    const result = await buildQuery(connection, query[i]);
    console.log(result);
    resultArray.push(result);
  }
  return resultArray;
});

I re-wrote this to use the same async/await syntax as below. You could use the callback syntax you're currently using, but you wouldn't be able to use the code below without changing it. I think the async/await syntax is more straightforward. It's also important that you have res.render as the last thing in your function (since it's signifying as a callback that you're done processing the request).

const controller = {
  retrieveTables: async function(req, res) {
    const database = req.params.database;
    const query = ["SHOW TABLES FROM " + database + ";"];

    const results = await connectDb(query, database);
    console.log('results from controller', results);
    res.render('dashboard', {data: results});
  }
}