Promise chain: parent promise is not waiting until child promise gets executed

Question

I fetch data from one mongoDB collection, in that response I got Id of another collection's data and fetch that and merge that in one object.

Here's my code, but it's not wait unitl child primise gets executed.

guide me with my code mistake.

 Courses.find({})
    .then( course => {
        //getting data from one collection
        let CoursePromises = course.map(
          key => {

            new Promise((resolve, reject) => {
                key.questions = []
                //getting data from another collection via Id fetched from first collection.
                let getQuestionsPromises = key.questionIds.map(
                  ques =>
                    new Promise((resolve, reject) => {
                             Questions.find({_id: ques._id})
                                .then(question => {
                                    resolve(question)
                                }).catch(err => {
                                    console.error("Error in  question ", err.message)
                                })
                    })
                )
                Promise.all(getQuestionsPromises).then((data) => {

                     key.questions.push(data)
                    console.log("getQuestionsPromises", key)
                })
                resolve(key)
            })
        })

        Promise.all(CoursePromises).then((data) => {
            console.log("CoursePromises") // here promise is now wait for exection done 
            res.send({ status: true, data: course })
          }
        ) 

I got first collection response like this:

{
    "status": true,
    "data": [
        {
            "_id": "5e3c1b683ac31f24da39e50a",
            "courseName": "Test",
            "duration": 1,
            "createdBy": "John Die",
            "__v": 0,
            "updatedAt": "2020-02-06T13:58:00.906Z",
            "createdAt": "2020-02-06T13:58:00.906Z",
            "isAssigned": false,
            "questions": []
            "questionIds": [
                {
                    "index": 1,
                    "_id": "5e3c1b683ac31f24da39e509"
                }
            ]
        }
    ]
}

with questionIds I fetch another recoed and put that reseponse in the existing object like this :

{
    "status": true,
    "data": [
        {
            "_id": "5e3c1b683ac31f24da39e50a",
            "courseName": "Test",
            "duration": 1,
            "createdBy": "John Die",
            "__v": 0,
            "updatedAt": "2020-02-06T13:58:00.906Z",
            "createdAt": "2020-02-06T13:58:00.906Z",
            "isAssigned": false,
            "questions": [
                [
                    [
                        {
                            "_id": "5e3c1b683ac31f24da39e509",
                            "index": 1,
                            "isVideo": false,
                            "questionType": "MCQ",
                            "question": "Is this a demo question?",
                            "title": "Question",
                            "description": "this is question description",
                            "link": "",
                            "createdBy": "Harsh",
                            "updatedBy": "",
                            "__v": 0,
                            "updatedAt": "2020-02-06T13:58:00.521Z",
                            "createdAt": "2020-02-06T13:58:00.521Z",
                            "options": [
                                {
                                    "one": "two"
                                }
                            ]
                        }
                    ]
                ]
            ],
            "questionIds": [
                {
                    "index": 1,
                    "_id": "5e3c1b683ac31f24da39e509"
                }
            ]
        }
    ]
}

Answer 1

You should follow pure async-await syntax when working with such complex structure. Also use .lean() to convert the course from mongoose object to a plain object.

Simplified code:

const course = await Courses.find({}).lean();
const coursePromises = course.map(async key => {
  key.questions = [];
  const getQuestionsPromises = key.questionIds.map(async ques => {
    const question = await Questions.find({ _id: ques._id });
    key.questions.push(question);
  });
  await Promise.all(getQuestionsPromises)
});
await Promise.all(coursePromises)
return res.send({ data: course })