How to do a find iterating in a array field - mongodb

Question

Hi i have a expressjs app using mongodb.
At first i find a tv by id on my "tvs" collection, i get it but now i want to find all user info from other collection "users".
This is my JSON for each collection:
tvs
{
"_id" : ObjectId("5203af83396d285ea2ecff8f"),
"brand" : "LG",
"comments" : [{
"user" : ObjectId("521dc636eda03d0f9cab3568"),
"text" : "Sold!"
}, {
"user" : ObjectId("521b2785eda03d0f9cab3566"),
"text" : "Nice TV"
}],
"model" : "47LS5600",
"price" : 499.0
}
users
{
"_id" : ObjectId("521b2785eda03d0f9cab3566"),
"name" : {
"first" : "Ruben",
"last" : "Montes"
}
}
And this is my code
var tvs = db.collection("tvs");
var users = db.collection("users");
exports.findById = function (req, res) {
var id = req.params.id;
tvs.findOne({'_id': new BSON.ObjectID(id)}, function (err, tv) {
users.find( { _id : tv.comments.user_id }).toArray(function (err, items) {
res.send( { tv: tv, users: items } );
});
})
}
I need to know how to iterate the comments array from tvs collection to get the the info user that post a comment
users.find( { _id : tv.comments.user_id })

Answer 1

You can do a bit more logic to efficiently grab the users as a batch using the $in operator.
var mongodb = require('mongodb')
, MongoClient = require('mongodb').MongoClient
, Server = require('mongodb').Server;
MongoClient.connect('mongodb://127.0.0.1:27017/test', function (err, db) {
if (err) throw err;
var tvs = db.collection('tvs');
var users = db.collection('users');
var userNames = {};
var tvId = new mongodb.ObjectID("5203af83396d285ea2ecff8f"); // hard-code
// find a TV
tvs.findOne({ _id : tvId }, function (err, tv) {
var allUserIds = [];
if (tv && tv.comments) {
// build a list of all user IDs used in comments
// this doesn't filter duplicates right now
allUserIds = tv.comments.map(function (comment) {
return comment.user;
});
}
// using the list of UserIds, grab all of them ...,
// and just return the name
users.find({_id: { $in: allUserIds }}, { name: 1 })
.toArray(function (err, users_list) {
// if we got some
if (users_list && users_list.length > 0) {
for(var i= 0, len = users_list.length; i < len ; i++ ) {
userNames[users_list[i]._id] = users_list[i].name;
}
console.log("All comments ========");
// now all the usernames are indexed in userNames by Id
for(var i= 0, len = tv.comments.length; i < len ; i++ ) {
// swap id for name
tv.comments[i].user = userNames[tv.comments[i].user];
console.log(tv.comments[i]);
}
db.close(); // done with everything for this demo
}
});
});
});
I've used find and $in with an array of all userIds found in the comments for a single "tv". By using $in, it significantly reduces the number of calls needed to MongoDB to fetch single User documents. Also, using the second parameter of find, I've reduced the returned fields to just be name.
FYI -- I did simplify your structure to just be 'name' rather than 'first' and 'last'. You certainly can change it to match your exact needs.