Get objects in a collection with an array of id's in mongoDB

Question

Bulding an API with node.js/express and mongoDB. I have two collections with like a many to many relation, Users and Items. I want to get all the Items that the User are following. The users Items is an array with ids referring to Items.

How do i query so i get all the Items thats in the useritems array?

Collections:

Users:

{
    email: "[email protected]",
    username: "johnny",
    useritems: ["51e101df2914931e7f000003", "51cd42ba9007d30000000001"]
}

Items:

{
       "name": "myitem",
       "description": "Description of item" 
       "_id": "51e101df2914931e7f000003"
}

{
       "name": "myitem2",
       "description": "Description of item2" 
       "_id": "51cd42ba9007d30000000001"
}

{
       "name": "myitem3",
       "description": "Description of item3" 
       "_id": "51e101df2914931e7f000005"
}

EDIT:

I updated the code. I now get the array of useritems ids based on the user id. Problem is when i try to send the items to array. Items is always empty. Is something wrong with my query?

exports.findItemsByUserId = function(req, res) {
    //var id = req.params.id;

   var userId = "51e6a1116074a10c9c000007"; //Just for testing

    db.collection('users', function(err, collection) {
        collection.findOne({'_id':new BSON.ObjectID(userId)}, function(err, user) {

            db.collection('items', function(err, collection) {

                console.log("user undefined? ",user);
                console.log("useritems ",user.useritems);
                collection.find({'_id': {'$in' : user.useritems}}).toArray(function(err, items) {
                    console.log("useritems ",user.useritems); // <--Gets me array with the ids
                    console.log("items ", items); //<--Empty
                    res.send(items);//Empty
                });
            });
        });
    });
};

Solution 1

Problem solved. Not the most fancy solution but it works. Just looping through some arrays. Guess there's some proper query for this.

exports.findItemsByUserId = function(req, res) {
   var id = req.params.id; //<--- was disabled, it will give an error when testing this code.

   var userId = "51e6a1116074a10c9c000007"; //Just for testing

    db.collection('users', function(err, collection) {
        collection.findOne({'_id':new BSON.ObjectID(id)}, function(err, user) {

            db.collection('items', function(err, collection) {
                var itemsArray = []

                var itemIds = user.useritems

                for (var i = 0; i < itemIds.length; i++) {
                    itemIds[i]

                    collection.findOne({'_id':new BSON.ObjectID(itemIds[i])}, function(err, item) {

                    itemsArray.push(item);

                    if(itemIds.length === itemsArray.length){

                        res.send(itemsArray);

                    }

                    });
                };

            });
        });
    });
};

Solution 2

Maybe this is more like it?

exports.findItemsByUserId = function(req, res) {
  var userId = "51e101df2914931e7f000003"; //Just for testing
  var user = db.users.find({"_id": userId});

  var items = db.items.find({'_id': {'$in' : user.useritems}});
  res.send(items.toArray());
};