expressjs mongodb find query loop with queries inside

Question

So I am using this query to create an array to pass on to ejs to generate table rows. but before I pass it, I have other queries like referencing ids or count. but I get undefined on count. I may have other queries inside.

router.get('/', function(req, res, next) {
    MongoClient.connect(MongoUrl, function(err, db) {

        var table_data = [];
        db.collection('categories').find().toArray(function(err, result) {

            for (var i = 0, len = result.length; i < len; i++) {
                var count;
                db.collection('articles').count({category_id: result[i].category_id}, function(err, count) {
                    count = count;
                });

                table_data[i] = {
                    "category_id" : result[i].category_id,
                    "category" : result[i].category,
                    "count" : count,
                    "date_last_imported" : result[i].date_last_imported,
                    "feed" : result[i].feed,
                    "url" : result[i].url,
                    "xml" : result[i].xml
                };

            }
            console.log(table_data);
            res.render('categories', {title:"Categories",table_data: table_data});
        });
    });
});

You aren't returning anything from the db.collection function. Try returning the value: return count; — Dandy
– Dandy, Commented Oct 13, 2016 at 1:50
var count = db.collection('articles').count({category_id:result[i].category_id}, function(err, count) {return count;}); doesnt work — Boy
– Boy, Commented Oct 13, 2016 at 1:54
Just looked this up, you actually want to do something like this: var count = db.collection('articles').count({category_id: result[i].category_id});. I've never done this, but the documentation implies this is best practice. — Dandy
– Dandy, Commented Oct 13, 2016 at 2:05

Sachin · Accepted Answer · 2016-10-13 02:48:08Z

1

It's because count is not yet fetched and you have pushed it in array that is undefined. If you have lot of queries inside queries then in that case you'll be trapped in callback hell. To get out of this scenario either use promise or use "async"module.

For reference: http://caolan.github.io/async/

For current scenario and to get idea why "count" is undefined:

router.get('/', function(req, res, next) {
    MongoClient.connect(MongoUrl, function(err, db) {

        var table_data = [];
        db.collection('categories').find().toArray(function(err, result) {

            for (var i = 0, len = result.length; i < len; i++) {
                var count;
                db.collection('articles').count({category_id: result[i].category_id}, function(err, count) {
                    count = count;
                table_data.push( {
                    "category_id" : result[i].category_id,
                    "category" : result[i].category,
                    "count" : count,
                    "date_last_imported" : result[i].date_last_imported,
                    "feed" : result[i].feed,
                    "url" : result[i].url,
                    "xml" : result[i].xml
                });
             if(table_data.length === result.length) {
               res.render('categories', {title:"Categories",table_data:  table_data});
             }
                 });



            }

        });
    });
});

UPDATE: To get out of inner queries.

router.get('/', function(req, res, next) {
    MongoClient.connect(MongoUrl, function(err, db) {
        var table_data = [];
        db.collection('categories').find().toArray(function(err, result) {
          async.each(result, function (user, cb) {
            async.waterfall([
              function (cb) {
                db.collection('articles').count({
                    category_id: result[i].category_id
                }, function(err, count) {
                    cb(err, count);
                });
              },
              function (count, cb) {
                //another query
                cb(err, count, secondquerydata);
              },
              .....
            ], function (err, count, secondquerydata, .....) {
              if(err) return callback(err);
              // here use your all query data
              table_data.push( {
                  "category_id": result[i].category_id,
                  "category": result[i].category,
                  "count": count,
                  "date_last_imported": result[i].date_last_imported,
                  "feed": result[i].feed,
                  "url": result[i].url,
                  "xml": result[i].xml
              });
              res.render('categories', {
                  title: "Categories",
                  table_data: table_data
              });
            })
          })

        });
    });
});

edited Oct 13, 2016 at 2:48

answered Oct 13, 2016 at 2:26

Sachin

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3 Comments

Boy Over a year ago

from async how can I pass it back and assign on the table_data array?

Boy Over a year ago

the table_data gets all the data now. but I'm not sure the res.render is in the correct place. it doesnt stop loading for me.

its me Over a year ago

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