cursor.toArray() returns a promise instead of array

Question

Currently using node 4.3.2 and mongo 2.6. I am attempting to get a whole collection (three documents currently in the collection). When i use this bit of code i run into an issue.

function checkUpdateTime(last_updated){
    var collection = db.collection(last_updated);
    collection.insert({a:1});
    updateTimes = collection.find({a:1}).toArray();
}
var updateTimes = [];
checkUpdateTime('last_updated');
console.log(updateTimes);

When this code is tun updateTimes is a promise and not the array i was hoping for. The goal is the edit the array then insert it back into the collection later.The insert statement works but the retrieval of the documents simply doesn't operate the way i was expecting. I have tried quite a few versions of this code but no dice.

I guess it boils down to me wondering why a promise is being returned?

robertklep · Accepted Answer · 2016-08-26 20:39:01Z

11

The MongoDB driver offers two options for handling asynchronous operations:

through callbacks that get passed by the caller
by returning a promise to the caller

When you don't pass a callback, like in your case, it will return a promise.

So you need to make a choice here. One choice that you can't choose is "make this code run synchronously", though.

I prefer promises:

function checkUpdateTime(last_updated){
  var collection = db.collection(last_updated);
  return collection.insert({ a : 1 }) // also async
                   .then(function() {
                     return collection.find({ a : 1 }).toArray();
                   });
}
checkUpdateTime('last_updated').then(function(updateTimes) {
  console.log(updateTimes);
});

You could always go a bit more fancy and use something like Promise.coroutine, that will make your code look a bit more synchronous (even though it isn't):

const Promise     = require('bluebird');
const MongoClient = require('mongodb').MongoClient;

let checkUpdateTime = Promise.coroutine(function* (db, last_updated){
  let collection = db.collection(last_updated);
  yield collection.insert({ a : 1 });
  return yield collection.find({ a : 1 }).toArray();
});

Promise.coroutine(function *() {
  let db = yield MongoClient.connect('mongodb://localhost/test');
  let updateTimes = yield checkUpdateTime(db, 'foobar');
  console.log(updateTimes);
})();

Or async/await, using Babel:

const MongoClient = require('mongodb').MongoClient;

async function checkUpdateTime(db, last_updated) {
  let collection = db.collection(last_updated);
  await collection.insert({ a : 1 });
  return await collection.find({ a : 1 }).toArray();
}

(async function() {
  let db = await MongoClient.connect('mongodb://localhost/test');
  let updateTimes = await checkUpdateTime(db, 'foobar');
  console.log(updateTimes);
})();

edited Aug 26, 2016 at 20:39

answered Aug 26, 2016 at 20:26

robertklep

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2 Comments

vp_arth Over a year ago

"you can't choose". He can. With async/await.

robertklep Over a year ago

@vp_arth still, that won't make it synchronous (the clue is in the name, "async" ;-). It may look like it, though, that's why I added it as an example :D

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