increase value in an array by index variable with mongoose mongodb nodejs

Question

I have an array of numbers :

readersNum : {type: [Number], default: []},

I would like to increase the value placed in the indexVariable by 1.

this works :

Posts.update({"readerID":readerID},{$inc:{"readersNum.0":1}},{upsert:true,safe:true},
 function(err){
 console.log(err);
 });

but the problem is i the value is not in a specific(constant) index it is a variable.. assuming the index variable is indexVariable and not 0, how should i increase it?

I tried also this :

Posts.findOne({"readerID":readerID },['readersNum'],function(err, readerID){
 readerID.readersNum[indexVariable] = readerID.readersNum[indexVariable]+1;
 try { 
 Posts.save(readerID,function(err) {
 if (err) {
 console.log("ERROR while saving reader num: " + err);
 }
 else {
 console.log('saved reader num successfully'); 
 }
 });
 }
 catch(e){
 console.log("ERROR while saving reader num: " + e);
 }
 });

executing the code above prints the changed value in the console and "saved reader num successfully" but the value in the database stays the same! I would really appreciate it if someone could tell me what am i doing wrong. Thank you!

Answer 1

Assuming that you know the index you could do this:

var update = {};
update['readersNum.' + index] = 1; // update now has property like `readersNum.3`
Posts.update({"readerID":readerID},{$inc: update},{upsert:true,safe:true}, function(err){ console.log(err); });