Mongoose promise never gets to .then()

Question

I am using q and I have multiple mongoose .exec() promises that never gets to the .then() part of the code, so never allow the q to resolve. Can't figure out why it never comes back.

    var defer = q.defer();
    var promises = [];
    console.log('Exams:', exams.length);
    for (var e=0; e<exams.length; e++) {
      console.log('Exams:', exams[e]._id);
      var newPromise = Pupilexam.find({ _exam: exams[e]._id }).populate('_user').exec()
        .then((pupils) => {
          console.log("Adding pupils", exams[e]._id);
          exams[e].pupils = pupils;
          resolve(exams[e]);
        })
        .catch((err) => {
          reject(err);
        });
      console.log(typeof newPromise);
      promises.push(newPromise);
      console.log("Promised pushed");
    }
    q.all(promises).then(function(data){
        console.log("q'd all");
        defer.resolve(res.status(200).json(exams));
    });
    return defer;

The Pupilexam.find().exec() never reaches the .then() so the promises never resolve and the defer never resolves. Why would the mongoose find not get to the .then()? What have I missed?

*** UPDATE ***

Even using the built in promises, we get the same issue. The Pupilexams.find() call never comes back.

    var promises = [];
    for (var e=0; e<exams.length; e++) {
      console.log('e:', e);
      console.log('Exam', exams[e]._id);
      var newPromise = Pupilexam.find({ _exam: exams[e]._id }).populate('_user').exec()
        .then((pupils) => {
          console.log("Adding pupils", exams[e]._id);
          exams[e].pupils = pupils;
        })
        .catch(handleError(res));
      promises.push(newPromise);
    }
    Promise.all(promises).then((exams) => {
      console.log(values);
      res.status(200).json(exams)
    });

With this method I also get a headers error on the call UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

** ADDITIONAL CODE REQUESTED **

function handleError(res, statusCode) {
    statusCode = statusCode || 500;
    return function(err) {
      console.log(err.message);
      res.status(statusCode).send(err);
    }; 
}

Answer 1

To answer the updated question regarding the Cannot set headers after they are sent to the client error. Looks like you send a response to the client inside your handleError function. Now, if more than one Pupilexam.find call fails, handleError would be invoked twice, resulting in the mentioned error.

You should move the catch-handler down to the Promise.all call:

const promises = [];
for (const exam of exams) {
    const newPromise = Pupilexam
        .find({ _exam: exam._id }).populate('_user').exec()
        .then((pupils) => {
            exam.pupils = pupils;
        });
    promises.push(newPromise);
}
Promise.all(promises)
  .then((exams) => {
    res.status(200).json(exams);
   }) 
  .catch(handleError(res));

Answer 2

I guess that you are indeed returning your promise but you are returning an empty json. There are 2 problems with your approach:

1. You are not returning from your then: should return pupils and it is returning undefined

2. You are logging values that I don't know what it is

    .then((pupils) => {
      console.log("Adding pupils", exams[e]._id);
      exams[e].pupils = pupils; 
      // you should return something // return pupils
    })
    promises.push(newPromise);             
   
    Promise.all(promises).then((exams) => {
    // ['undefined', 'undefined', ...]
          console.log(values);
          res.status(200).json(exams)
        });
   

Answer 3

Looks like the answer was that on these two lines the exams[e] is not in scope, because by the time the promise comes back the loop has moved on, so e is wrong and gets too high so it was erroring.

      console.log("Adding pupils", exams[e]._id);
      exams[e].pupils = pupils;

Only discovered that when I read @eol's message about the catch and decided to catch it properly and output.

Answer 4

it is Look from your code.

    //(async) function
    for (var e of exams) {
        try {
         const pupils = await  Pupilexam.find({ _exam: exams[e]._id 
                }).populate('_user').exec().lean()
          e.pupils = pupils
             
        }catch((err){
          //handleError
        };
    }
res.status(200).json({data: exams})
    

maybe that will show you how match are you wrong