MongoDB $and query not working with variable query-string

Question

I'm using the MongoDB $and in combination with $lt and $gt like this:

  var queryString0 = 'data.' + req.body.data[0].keyname;

  console.log(queryString0);
  //Prints 'data.price'

  Company.find({ $and : [{ queryString0 : { $lt : 200, $gt : 100}}]}, {name: 1, queryString0 : 1} , function(err, result) {
    if (err) {
      throw err;
    }

    res.send(result);

  });

The problem is that this gives me no results while I use the queryString0 to retrieve the data, however, it gives me the desired results if I use a static string like this:

  var queryString0 = 'data.screener.' + req.body.data[0].keyname;

  console.log(queryString0);
  //Prints 'data.price'

  Company.find({ $and : [{ 'data.price' : { $lt : 200, $gt : 100}}]}, {name: 1, 'data.price' : 1} , function(err, result) {
    if (err) {
      throw err;
    }

    res.send(result);

  });

This doesn't make any sense to me, as the same query-string should be provided in both cases.

Please help me figuring out what is wrong with my code.

EDIT : The reason I used $and is because I will also need to check if several fields match my other conditions, like this:

  var queryString0 = 'data.screener.' +req.body.data[0].keyname;
  var queryString1 = 'data.screener.' +req.body.data[1].keyname;

  Company.find({ $and : [
    {queryString0 : {$lt : req.body.data[0]["max"], $gt : req.body.data[0]["min"]}},
    {queryString1 : {$lt : req.body.data[1]["max"], $gt : req.body.data[1]["min"]}}
  ] },{ name : 1, queryString0 : 1, _id : 0 }, function(err, result) {
    if (err) {
      throw err;
    }

    res.send(result);

  })

Answer 1

And you started of with the right concept my constructing something in code, but forgot that JavaScript "stringifies" object keys as well. So there is a different notation for that.

Plus you also not no need $and here at all:

var queryString0 = 'data.screener.' + req.body.data[0].keyname;

var query = {};
query[querystring0] = { $lt : 200, $gt : 100};

var projection = { name: 1 };
projection[querystring0] = 1;

Company.find(query,projection,function(err,result) {
   // work in here
});

So the "bracketed" notation is how you use variable contents as the name of a key. And "all" MongoDB query arguments are "and" conditions anyway, so you would rarely need to wrap them with an $and , unless you need "two" or more conditions for the same field.