How to populate array inside array of object in mongoose - reactjs

Question

i have a classShcema,inside my marksSchema i have class property i.e populated from classSchema.note that classSchema has subject and it is populate from subjectSchema.
const classSchemaDef = new mongoose.Schema({
classes:{type:Number},
subject:[{
type:mongoose.Types.ObjectId,
ref:'subject',
require:true,
default:null
}],
section:[String],
created_by:commonSchema.created_by,
},commonSchema.Trigger)
below is the Marksschema that has class in it. the class is populated but subject is not being populate.
const MarksSchemaDef = new mongoose.Schema(
{
student: {
type: mongoose.Types.ObjectId,
required: true,
ref: "student",
default: null,
},
marks_obtain: [
{
class: {
type: mongoose.Types.ObjectId,
ref: "class",
required: true,
default: null,
},
subject: {
type: mongoose.Types.ObjectId,
ref: "subject",
},
marks: {
type: Number,
required: true,
},
created_by: commonSchema.created_by,
},
],
},
commonSchema.Trigger
);
below is the code to retrive data from marksModel and populate the data.
getAllMarks = async (skip, limit) => {
return await MarksModel.find()
.skip(skip)
.limit(limit)
.populate("student")
.populate("marks_obtain.class")
.populate("marks_obtain.class.subject")
.populate("marks_obtain.subject");
}
below is the response data i got
{
"_id": "63ce6eb551a5815f8662fd87",
"student": {
"parentname": {
"fullname": "mohan shrestha",
"phone": 868594
},
"_id": "63c80a2912d13e1af652df71",
"studentname": "mogina shrestha",
"class": 8,
"image": "localhost:3000/pic.jpg",
"address": "dhanusmode",
"phone": 2675442,
"section": "a",
"created_by": null,
"__v": 0
},
"marks_obtain": [
{
"class": {
"_id": "63a6a17590f43440d694cedb",
"classes": 8,
"subject": [
"63a42fbd14407d172f07586d",
"63a42fd814407d172f075873"
],
"section": [
"a",
"b",
"c"
],
"created_by": null,
"__v": 0
},
"subject": {
"_id": "63a42fd814407d172f075873",
"subjectname": "grammer",
"__v": 0
},
"marks": 55,
"created_by": null,
"_id": "63ce6eb551a5815f8662fd88"
}
],
"__v": 0
}
],