nodejs mongodb $in [array] not working if array is a variable - node.js

Question

This is very strange to me. If put the array onlyIds in the aggregation query of my db I get no results. If I however put the content of onlyIds that I get printed from line 5 which looks like:
["52e953942a13df5be22cf792","52e953942a13df5be22cf793","52e953942a13df5be22cf797"...]
Then it works. But not if I use the variable.
This function:
var onlyIds = [];
for (var i = 0; i < users.length; i++) {
onlyIds.push(users[i]._id);
}
console.log("ids: " + JSON.stringify(onlyIds)); <---------- not empty
db.collection('posts', function(err, collection) {
collection.aggregate([
{$match: {user_id: {$in: onlyIds}}}, <------- not working
{$match: {created:{$gte: 0}}},
{$sort:{"created": -1}},
{$skip: req.body.skip},
{$limit: req.body.limit}
],
function(err, posts) {
var errorNo, content, message;
if (err) {
errorNo = resSend.errorDB;
message = JSON.stringify(err);
} else {
errorNo = resSend.errorNo;
content = posts;
message = "";
--> console.log(JSON.stringify(posts));
}
resSend.sendResponse(res, resSend.errorNo, content, message);
});
});
So in short, why does this work:
{$match: {user_id: {$in: ["52e953942a13df5be22cf792","52e953942a13df5be22cf793","52e953942a13df5be22cf797"...]}}}
and this doesn't:
{$match: {user_id: {$in: onlyIds}}}
And the line that does not work, works perfectly in another function. Any ideas or enlightenments?
EDIT:
Switching to find and using the below answer like this:
collection.find({'user_id': {$in: onlyIdsX}}).toArray(function(err, posts)
does not work either.
ANSWER:
As the selected answer indicates below is when the variable you search for is an ObjectId or a string. For anyone else, make sure that the variable in the db is the same type as the one you try to match it with. In my case both were supposed to be strings, but one's in "onlyIds" were ObjectIds.

Answer 1

Try following code to modify your loop:
var ids = ["52e953942a13df5be22cf792","52e953942a13df5be22cf793","52e953942a13df5be22cf797"];
var obj_ids = [];
for (var i = 0; i < users.length; i++) {
obj_ids.push(new ObjectID(users[i]._id.toString()));
var obj_ids.push(users[i]._id); // <== This will not work if your DB has _id : ObjectID("xyz") [i.e. you are not overiding defaults]
}
And you should include var ObjectID = require('mongodb').ObjectID; into your code.
You should use .toArray(function(err,.. (Not in your case since you used aggregation framework).This will also cause issue if you are not using findOne() (For more info on this here is the link)
Following is the example which spots the issue (in comments) & working Code:
var mongo = require('mongodb'),
Server = mongo.Server,
Db = mongo.Db,
ObjectID = require('mongodb').ObjectID;
var BSON = require('mongodb').BSONPure;
var server = new Server('localhost', 27017, {
auto_reconnect: true
});
var MongoClient = require('mongodb').MongoClient
//let id = your _id, smth like '6dg27sh2sdhsdhs72hsdfs2sfs'...
var users = ["52e953942a13df5be22cf792","52cbd028e9f43a090ca0c1af","52e953942a13df5be22cf797"];
var obj_ids = [];
for (var i = 0; i < users.length; i++) {
obj_ids.push(new ObjectID(users[i].toString()));
//obj_ids.push(users[i]._id); // <== This will not work if your DB has _id : ObjectID("xyz") [i.e. you are not overiding defaults]
}
MongoClient.connect('mongodb://127.0.0.1:27017/YourDBName', function(err, db) {
console.log('err' + err);
db.collection('posts', function(error, collection) {
//collection.find({_id:{$in: users}}),function(err, docs) { //This will not work
collection.find({_id:{$in: obj_ids}}).toArray(function(err, docs) {
console.log("Printing docs from Array. count " + docs.length);
docs.forEach(function(doc) {
console.log("Doc from Array ");
console.dir(doc);
});
});
});
});