Accessing db.collection outside of connection callback

Question

How do I make a var / object / module with a reference to a specific collection of the connected DB?

How do I make a module to export a reference to a specific collection to be used elsewhere:

Module connect-to-db.js

var MongoClient = require('mongodb').MongoClient;
var assert = require('assert');
var url = 'mongodb://<user>:<password>@..shard..mongodb.net:<port>/<collection>?ssl=true&replicaSet=..shard..&authSource=admin';

// This part works
MongoClient.connect(url, function(err, db) {
   assert.equal(null, err);
   console.log('Connected to external MongoDB server');
   // db.close(); // not called to keep connection open

   // How do I return db to the outside of this callback's scope 
   //   to exports.user?  
});

// This is the part that's not working
// How do I access db.collection('users').find() from outside this module
module.exports.users = MongoClient;
// this should return a reference to db.collection('users') in exports.users

// where should I return

Then be able to query / write to that collection from anywhere (using dBconnection.users as below):

Main App server.js

var express = require('express');
var app     = express();
var dBconnection = require('./connect-to-db');
// is dBconnection is a reference to MongoClient.connection ?
// is dBconnection.users a reference to db.collection('users') ?

app.get('/db', function(req, res){

    // From here does not work
    records = dBconnection.users.find();
    // runtime error: cannot use .find() of undefined 
    // OR runtime error: find() is not a function of users 

    // Testing the query
    res.setHeader('Content-Type', 'text/plain');
    res.write('Attempting to display records\n');
    res.end(JSON.stringify(records, null, 2));
});

app.listen(3000, function () {
    console.log('Listening at port 3000')
});

In the end I want to make the module maintain the connection (even if dropped, maybe adding a .status property to exports to check the status) so that I can access it anytime elsewhere in server.js / other modules / other routes / views

How about exporting a `Promise` and using `await`: `const db = await require('./connect-to-db')` or `require('./connect-to-db').then((db) => {...})`? Or expose a function: `require('foxes').myConnect().then((foxes) => {...})` — ilyaigpetrov, Mar 24 '17 at 16:16
Thanks for the suggestion - I'll have to read up a bit more on `promises` and `await` first :) — Carl J du Preez, Mar 24 '17 at 16:24

score 0 · Answer 1 · answered Mar 24 '17 at 16:18

Nothing that is immediately available cannot be exported by a module in Node, because Node modules exports and the require() function are synchronous. (See this answer for more info: javascript - Why is there a spec for sync and async modules?)

What you can export, though, is a promise of the thing that you need in other requiring modules, like the db.collection('users').

For example you can do something like:

module.exports.users = new Promise((resolve, reject) => {
    someAsyncFunction((err, data) => {
        if (err) return reject(err);
        resolve(data);
    });
});

And then other modules will be able to use it like:

require('./the/module').users.then(users => {
  // use the users
});

or inside async functions:

let users = await require('./the/module').users;
// use the users

But you will not be able to export directly the value that is not available immediately when the module is required.

Thanks for pointing me in the right direction - I'll have to read up a bit more on `promises` and `await` and the link you provided first - I'm still a bit new to node.js — Carl J du Preez, Mar 24 '17 at 16:31

Accessing db.collection outside of connection callback

1 Answers1