35

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.

Mike Pateras
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3 Answers3

31

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 :)

Amol M Kulkarni
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  • I coded a 5 line script with mongo-sync and it failed, even though it matched their test code nearly perfectly. It seems to have bugs. – jcollum Jul 23 '13 at 17:36
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    @jcollum : Can you please describe the exact issue you had? because its working for me with no major issues.. If you are sure its a bug in module you can raise a new issue on [Repo](https://github.com/olegp/mongo-sync/issues) – Amol M Kulkarni Jul 24 '13 at 05:46
  • I submitted a bug. Apparently you have to delete the fibers modules from node_modules in the mongo-sync lib. Looks like a packaging problem. – jcollum Jul 24 '13 at 17:17
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    If it's dependent on node-fiber then it's not synchronous – Esailija Apr 18 '14 at 09:45
  • Unstable package. mongo-sync gives assertion failure at the time of initialization only. – Atul Apr 11 '22 at 07:09
22

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.

JohnnyHK
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  • Thanks Johnny for this workaround! I wish there was a simple way out of the box... it is frustrating even to write a simple `if_exists()` function... Btw, if anybody knows an easier way, or an update from the driver, please post it here. – Logan Dec 14 '12 at 12:38
  • You can always use an async library, to avoid the identation of "doom" https://github.com/caolan/async That will make the code more readable and nice. – ElHacker Dec 15 '12 at 03:25
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    @Logan I wouldn't call it a "workaround", that is how node is designed to work. – PeterT Mar 08 '15 at 02:44
  • returned ```thing``` is a '[object Promise]'. How can we read data inside promise object? (without ```.then``` callback) – Rishitha Minol May 29 '18 at 10:36
  • @RishithaMinol You can `await` the promise from within another `async` function: `let thing = await getAThing();` – JohnnyHK May 29 '18 at 13:53
  • I am trying to move away from the callback function model where it is easy to handle an error or success result, ie `function(err, result) { if (err) { res.send(err) } else { // do things }` - how do you handle/view an error that comes back from `let thing = await db.collection.findOne(query)`? – user1063287 Nov 13 '18 at 14:33
  • @user1063287 With `await` you have to use try/catch blocks for error handling. – JohnnyHK Nov 13 '18 at 14:46
  • @JohnnyHK - like this? `try { var bob = await getAThing() } catch(err) { console.log("mongodb query error is here: " + err ) }` – user1063287 Nov 13 '18 at 14:52
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    @user1063287 Yes. See [here](https://stackoverflow.com/questions/44663864/correct-try-catch-syntax-using-async-await) for more examples. – JohnnyHK Nov 13 '18 at 15:09
3

While it's not strictly synchronous, a pattern I've repeatedly adopted and found very useful is to use co and promisify yield on asynchronous functions. For mongo, you could rewrite the above:

var query = co( function* () {

    var db = new mongo.Db("mydatabase", server, {});
    db = promisify.object( db );
    db = yield db.open();

    yield db.authenticate("myuser", "mypassword");

    var collection = yield db.collection("Things");
    return yield collection.findOne( { name : "bob"} );

});

query.then( result => {

} ).catch( err => {

} );

This means:

  1. You can write "synchronous"-like code with any asynchronous library
  2. Errors are thrown from the callbacks, meaning you don't need the success check
  3. You can pass the result as a promise to any other piece of code
Hugheth
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