What is the right way to make a synchronous MongoDB query in Node.js?

Question

I'm using the Node.JS driver for MongoDB, and I'd like to perform a synchronous query, like such:

function getAThing() {     var db = new mongo.Db("mydatabase", server, {});      db.open(function(err, db)     {         db.authenticate("myuser", "mypassword", function(err, success)         {             if (success)             {                 db.collection("Things", function(err, collection)                 {                     collection.findOne({ name : "bob"}, function(err, thing)                     {                                                    return thing;                     });                 });             }         });     }); } 

The problem is, db.open is an asychronous call (it doesn't block), so the getAThing returns "undefined" and I want it to return the results of the query. I'm sure I could some sort of blocking mechanism, but I'd like to know the right way to do something like this.

Answer 1

ES 6 (Node 8+)

You can utilize async/await

await operator pauses the execution of asynchronous function until the Promise is resolved and returns the value.

This way your code will work in synchronous way:

const query = MySchema.findOne({ name: /tester/gi }); const userData = await query.exec(); console.log(userData) 

---

Older Solution - June 2013 ;)

Now the Mongo Sync is available, this is the right way to make a synchronous MongoDB query in Node.js.

I am using this for the same. You can just write sync method like below:

var Server = require("mongo-sync").Server; var server = new Server('127.0.0.1'); var result = server.db("testdb").getCollection("testCollection").find().toArray(); console.log(result); 

Note: Its dependent on the node-fiber and some issues are there with it on windows 8.

Happy coding :)

Answer 2

There's no way to make this synchronous w/o some sort of terrible hack. The right way is to have getAThing accept a callback function as a parameter and then call that function once thing is available.

function getAThing(callback) {     var db = new mongo.Db("mydatabase", server, {});      db.open(function(err, db)     {         db.authenticate("myuser", "mypassword", function(err, success)         {             if (success)             {                 db.collection("Things", function(err, collection)                 {                     collection.findOne({ name : "bob"}, function(err, thing)                     {                                db.close();                                             callback(err, thing);                     });                 });             }         });     }); } 

Node 7.6+ Update

async/await now provides a way of coding in a synchronous style when using asynchronous APIs that return promises (like the native MongoDB driver does).

Using this approach, the above method can be written as:

async function getAThing() {     let db = await mongodb.MongoClient.connect('mongodb://server/mydatabase');     if (await db.authenticate("myuser", "mypassword")) {         let thing = await db.collection("Things").findOne({ name: "bob" });         await db.close();         return thing;     } } 

Which you can then call from another async function as let thing = await getAThing();.

However, it's worth noting that MongoClient provides a connection pool, so you shouldn't be opening and closing it within this method. Instead, call MongoClient.connect during your app startup and then simplify your method to:

async function getAThing() {     return db.collection("Things").findOne({ name: "bob" }); } 

Note that we don't call await within the method, instead directly returning the promise that's returned by findOne.