I want to populate answers(comments) on my post . But on populating it becomes null while before that it stores fine of storing Id's of answer.

My schema of post

var doubtsSchema = new mongoose.Schema(
 title : String,
 content : String,
 tags : String,
 created : 
 type : Date,
 default : Date.now,
 author : 
 id : 
 type : mongoose.Schema.Types.ObjectId, 
 ref : "User",
 username : String,
 answers : [
 type : mongoose.Schema.Types.ObjectId,
 ref : "Answers"]);

module.exports = mongoose.model("Doubts",doubtsSchema);

My schema of answer

var answersSchema = new mongoose.Schema(
 content : String,
 created : 
 type : Date,
 default : Date.now,
 author : 
 id : 
 type : mongoose .Schema .Types . ObjectId, 
 ref : "User",
 username : String,
 likes_count : Number);


module.exports = mongoose.model("Answers",answersSchema);

Populate is not working

Doubts.findById(req.params.id).populate('answers').exec(function(err,foundDoubt) 
 if(err) 
 console.log(err);
 else 
 console.log("here");
 console.log(foundDoubt);
 res.render("doubts/show",doubt : foundDoubt);
 
);

I made a simple example, it works

const mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/test", useNewUrlParser: true);

const UserSchema = new mongoose.Schema(
 name: String,
 comments: [type: mongoose.Schema.Types.ObjectId, ref: "Comments"]
);

const CommentSchema = new mongoose.Schema(
 content: ""
);

const Users = mongoose.model("Users", UserSchema);
const Comments = mongoose.model("Comments", CommentSchema);

// Adding data
Promise.all([
 new Comments(content: "test 1").save(),
 new Comments(content: "test 2").save(),
 new Comments(content: "test 3").save()
]).then(result => 
 result = result.map(r => r._id);
 new Users(name: "test", comments: result).save().then(user => 
 // Getting with populate
 Users.findById(user._id).populate("comments").then(console.log);
 )
).catch(console.error);

In console:

 comments:
 [ _id: 5c979d9dedc0b1db90fe81dd, content: 'test 1', __v: 0 ,
 _id: 5c979d9dedc0b1db90fe81de, content: 'test 2', __v: 0 ,
 _id: 5c979d9dedc0b1db90fe81df, content: 'test 3', __v: 0 ],
 _id: 5c979d9dedc0b1db90fe81e0,
 name: 'test',
 __v: 0 

Maybe it will help to find the error