Save document after subdocument been updated mongoose, mongodb

Question

I want to update a subdocument's value and after that save the main document. So the changes to the subdocument gets saved to the database. Here is my code ATM, I find the correct subdocument, that is not the problem I guess. But it doesn't save the changes.

 projectModel.findById({_id: req.body.projectId})
  .then(function(doc){

    question = doc.qp.questions.filter(q => {return q._id == req.body.questionId})[0];

    var ans = question.answers.filter(a => {return a._id == req.body.answerId})[0];
    if(ans)
    {console.log(ans)}
    ans.value = req.body.answerValue;

    doc.save().then(res.send({questionId: req.body.questionId,answerId: req.body.answerId,answerValue: req.body.answerValue}))

  })
  .catch(error => console.log(error))

my object is kinda complex, and it has to be.. it looks something like this:

project={
         prop,
         prop,
         prop, 
         qp:{
             prop,
             prop,
             prop, 
             questions:[
                        question:{
                                  prop,
                                  prop,
                                  prop, 
                                  answers:[
                                           answerModels<--!this i want to find and edit!-->
                                          ]
                                 }
                        ]
            }

}

Answer 1

You should consider FindOneAndUpdate function of Mongoose.

This will allow you to find your document, edit it and save it back in your MongoDB.

Answer 2

I solved it like this:

projectModel.findById({_id: req.body.projectId},function (err, doc)
  {
    if(err)
    {
      console.log(err)
      return;
    }
    for (i in doc.qp.questions) {
      if(doc.qp.questions[i]._id == req.body.questionId)
      {
        for(y in doc.qp.questions[i].answers)
        {
          if(doc.qp.questions[i].answers[y]._id == req.body.answerId)
          {
            var test = doc.qp.questions[i].answers[y].value
            console.log(test);
            var query = "qp.questions"+i+"answers."+y+".value";

            doc.update({_id: req.body.questionId}, { '$set': {query : req.body.answerValue} });
            console.log(doc.qp.questions[i].answers[y].value);
            doc.save().then(() => res.send({questionId: req.body.questionId,answerId: req.body.answerId,answerValue: req.body.answerValue}))
          }
        }
      }
    }
  })
  .catch(error => console.log(error))