Why can't you modify the data returned by a Mongoose Query (ex: findById)

Question

When I try to change any part of the data returned by a Mongoose Query it has no effect.

I was trying to figure this out for about 2 hours yesterday, with all kinds

Answer 1

I think the Mongoose documentation doesn't make this clear enough, but the data returned in the query (although you can res.send() it) is actually a Mongoose Document object, and NOT a JSON object. But you can fix this with one line...

Survey.findById(req.params.id, function(err, data){
    var len = data.survey_questions.length;
    var counter = 0;

    var data = data.toJSON(); //turns it into JSON YAY!

    _.each(data.survey_questions, function(sq){
        Question.findById(sq.question, function(err, q){
            sq.question = q;

            if(++counter == len) {
                res.send(data);
            }
        });
    });
});

Answer 2

For cases like this where you want a plain JS object instead of a full model instance, you can call lean() on the query chain like so:

Survey.findById(req.params.id).lean().exec(function(err, data){
    var len = data.survey_questions.length;
    var counter = 0;

    _.each(data.survey_questions, function(sq){
        Question.findById(sq.question, function(err, q){
            sq.question = q;

            if(++counter == len) {
                res.send(data);
            }
        });
    });
});

This way data is already a plain JS object you can manipulate as you need to.