Getting Mongo database values in forEach loop

Question

I have two schemas Teacher and Student

StudentSchema = new mongoose.Schema({
email:{type:String, index: {unique:true}}
name:{type:String},
marks:[{
subject:{type:String,
marks:{type:Number}
}]
})


TeacherSchema = new mongoose.Schema({
email:{type:String, index: {unique:true}}
name:{type:String},
students:[{
email:{type:String},
registerationDate:{type:Date}
}]
})

I have an API where I get teacher's email id and have to respond with marks and name of the students registered to that particular teacher.

For this, I'm using this code

var teacher = await Teacher.findOne({"email":req.body.email})

teacher.students.forEach(function(students){
let student = Student.findOne({"email":students.email})
console.log(student)   // to watch the result
})

I want to get the complete Student schema in my student variable so that I can use the data of the students.

But I'm not getting the desired output because I can't user await along with Student.findOne.

Like this

let student = await Student.findOne({"email":students.email})

I'm getting a Query object as a result.

Can anyone suggest any way to use await in the loop or any other way to get my desired output?

Node crashes if I use await anywhere inside my loop, so solutions answered elsewhere to use async/await in loop is not solving my problem.

Possible duplicate of [Using async/await with a forEach loop](https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop) — DragonBorn, Dec 24 '18 at 06:59
@DragonBorn I have tried solutions answered by the people in that post but node crashes if i use await anywhere inside my loop, so it doesn't solve my problem. — harshit raghav, Dec 24 '18 at 07:05
And what error did you get when it crashed and did you use `for of` instead of `forEach` ? — DragonBorn, Dec 24 '18 at 07:08
@DragonBorn I got this error :TypeError: this.$__path is not a function — harshit raghav, Dec 24 '18 at 07:34
Can you include the code that you tried from the other post which gave this error. Please include the whole function starting from your `async`. — DragonBorn, Dec 24 '18 at 07:46
@DragonBorn Whenever I use await inside for loop or forEach loop I get this error. — harshit raghav, Dec 24 '18 at 07:53
You cannot use `await` inside `forEach` or `for` loop, you have to use `for of` loop like this `for (let student of teacher.students) { //code here }` — DragonBorn, Dec 24 '18 at 08:01
@DragonBorn Yeah, it worked thanks a lot buddy. But I didn't see this on that other question. — harshit raghav, Dec 24 '18 at 08:57

Vinesh Goyal · Answer 1 · 2018-12-24T09:12:03.660

Note: There is no native synchronous API for Mongoose queries but you can chain the queries.

Try this one;

Teacher.findOne({"email":req.body.email}, (err, teacher)=>{
    if( !err ) {
      let studentEmailIds = [];
      teacher.students.forEach( (students) => {
          studentEmailIds.push(students.email);
      });
      Student.find({'email': {"$in": studentEmailIds } }, (err, students)=>{     
         if(err){
          //error handle
         }else{
          console.log(students)
          //res.json(students);
         }
      });
    }
})

Alexander Chef · Answer 2 · 2018-12-24T08:21:47.103

0

Instead of forEach you can use find function of mongoose like below:

teacher.find({}, function (err, teachers) {
  // now you can use teachers.forEach here for getting students
  console.log(teachers.students)
});

I suppose you are trying to get all Teachers with their students.

edited Dec 24 '18 at 08:21

answered Dec 24 '18 at 07:19

Alexander Chef

378
1
2
13

Students should be in teachers, Can you put debug on the commented line and send me what is inside in teachers. – Alexander Chef Dec 24 '18 at 07:54
I don't get it which one are you asking, the one inside my question or the one in your answer? – harshit raghav Dec 24 '18 at 08:02
I mean try my answer and watch teachers. teacher.find({}, function (err, teachers) { // now you can use teachers.forEach here for getting students console.log(teachers.students) }); – Alexander Chef Dec 24 '18 at 08:19
Node crashed without any error message [nodemon] app crashed - waiting for file changes before starting... – harshit raghav Dec 24 '18 at 08:32

ibn_Abubakre · Answer 3 · 2018-12-24T08:13:42.483

0

Based on my understanding of your question, this should help

Teacher.find({}, (error, allTeachers) => {
  if (error) {
    // Handle Error
  } else {
    allTeachers.forEach(teacher=> {
      let student = Student.findOne({"email":teacher.email})
    });
  }
})

edited Dec 24 '18 at 08:13

answered Dec 24 '18 at 07:27

ibn_Abubakre

112
7

My Node keeps on crashing using your code without displaying the error message inside the if condition. – harshit raghav Dec 24 '18 at 07:51
I made a syntax error, it's fixed now, you could try it – ibn_Abubakre Dec 24 '18 at 08:00
It still crashes the same. – harshit raghav Dec 24 '18 at 08:08
I think it's cos i used teacher.findOne instead of Teacher.findOne. Try changing that. Also, what error do you get when it crashes? – ibn_Abubakre Dec 24 '18 at 08:13
It doesn't show any error it straight away crashes. – harshit raghav Dec 24 '18 at 08:32
I have console.log inside the if – harshit raghav Dec 24 '18 at 08:57
And are you getting the log?? – ibn_Abubakre Dec 24 '18 at 09:07

DragonBorn · Accepted Answer · 2018-12-24T09:46:57.470

You cannot use await inside a forEach loop. You have to use a for .. of loop instead.

A forEach loop fires of multiple asynchronous requests but the function immediately returns after that. It does not wait for your promise to resolve while a for .. of loop does.

Change your code to:

for (let students of teacher.students) {
  let student = Student.findOne({"email":students.email})
  console.log(student)   // to watch the result
});

Getting Mongo database values in forEach loop

4 Answers4