Node.js MongoDB insert error

Question

function uploadVote(voter, vote){
MongoClient.connect(url, function(err, db){
    if (err) throw err;

    db.collection('testvote', function(err, collection) {
        collection.find({"psid": voter}).toArray(function(err, result) {
            if(!result[0]){
                db.collection("testvote").insert({psid: "a", choice:0}, function(err, res) {
                    if (err) throw err;

                    console.log('insertone')
                    sendTextMessage(". ")
                });
            } else if(!result[0].choice){
                db.collection("testvote").update({psid: voter}, {$set: {'choice': vote}}, function(err, res){
                    if (err) throw err; 
                    sendTextMessage(". ")
                })
            } else{
                sendTextMessage(voter, "!");
            }

        });
        db.close(); 
    });
})
}

Hi, I'm developing a facebook messenger that let users vote something. In the code above, I see no syntax error at all. But when this function is executed it keeps making a error:

/app/node_modules/mongodb/lib/utils.js:123
2017-05-18T12:09:19.856188+00:00 app[web.1]:     process.nextTick(function() { throw err; });
2017-05-18T12:09:19.856188+00:00 app[web.1]:                                   ^
2017-05-18T12:09:19.858765+00:00 app[web.1]:     at ReplSet.insert (/app/node_modules/mongodb/lib/replset.js:398:18)
2017-05-18T12:09:19.858761+00:00 app[web.1]: MongoError: topology was destroyed
2017-05-18T12:09:19.858764+00:00 app[web.1]:     at ReplSet.insert (/app/node_modules/mongodb-core/lib/topologies/replset.js:1025:47)
2017-05-18T12:09:19.858766+00:00 app[web.1]:     at executeCommands (/app/node_modules/mongodb/lib/bulk/ordered.js:455:23)
2017-05-18T12:09:19.858766+00:00 app[web.1]:     at OrderedBulkOperation.execute (/app/node_modules/mongodb/lib/bulk/ordered.js:508:10)
2017-05-18T12:09:19.858767+00:00 app[web.1]:     at bulkWrite (/app/node_modules/mongodb/lib/collection.js:664:8)
2017-05-18T12:09:19.858768+00:00 app[web.1]:     at Collection.insertMany (/app/node_modules/mongodb/lib/collection.js:534:44)
2017-05-18T12:09:19.858769+00:00 app[web.1]:     at /app/index.js:121:41
2017-05-18T12:09:19.858768+00:00 app[web.1]:     at Collection.insert (/app/node_modules/mongodb/lib/collection.js:836:15)
2017-05-18T12:09:19.858770+00:00 app[web.1]:     at handleCallback (/app/node_modules/mongodb/lib/utils.js:120:56)
2017-05-18T12:09:19.858771+00:00 app[web.1]:     at /app/node_modules/mongodb/lib/cursor.js:860:16

Please help me to solve this.

Answer 1

The database calls will be asynchronous meaning that the db.close() line will be run before the code inside the callbacks.

You need to make sure that db.close() only gets called after you are done with the connection. You could either just put it at the end of each code branch (i.e. after each case of sendTextMessage()) or wrap the whole thing in a promise and call resolve in those locations instead.

Here is a version of your code wrapped in a promise. The resolve() method is only called when you know there are no more requests to be made. You could also not wrap it in a promise and place a db.close() statement in each place that I have put a resolve() statement.

function uploadVote(voter, vote){
  MongoClient.connect(url, function(err, db){
      if (err) throw err;

      new Promise((resolve, reject) => {
          db.collection('testvote', function(err, collection) {
              collection.find({"psid": voter}).toArray(function(err, result) {
                  if(!result[0]){
                      db.collection("testvote").insert({psid: "a", choice:0}, function(err, res) {
                          if (err) throw err;

                          console.log('insertone')
                          sendTextMessage(". ")
                          resolve();
                      });
                  } else if(!result[0].choice){
                      db.collection("testvote").update({psid: voter}, {$set: {'choice': vote}}, function(err, res){
                          if (err) throw err; 
                          sendTextMessage(". ")
                          resolve();
                      })
                  } else{
                      sendTextMessage(voter, "!");
                      resolve();
                  }

              });
          });
      }).then(() => db.close());
  })
}