I am creating a student management system. I am using nodejs, expressjs, ejs and mongoose.
Here is the code for Student model:

const mongoose = require('mongoose')
const studentSchema = mongoose.Schema({
    name: {type: String, required: true},
    batchInfo: [{
        batch_id: {type: mongoose.Schema.Types.ObjectId, ref: "Batch", required: true},
        studentID: {type: String, required: true, unique: true},
    }],
})
const Student = mongoose.model('Student', studentSchema, 'students')
module.exports = Student

Here, in the batchInfo array there should be different batch’s info.

Here is the code for Batch model:

const mongoose = require('mongoose')
const batchSchema = mongoose.Schema({
    academicYear: {type: Number, required: true},
})
const Batch = mongoose.model('Batch', batchSchema, 'batches')
module.exports = Batch

Here is the express code:

const router = require('express').Router()
const mongoose = require('mongoose')
const Batch = require("./models/Batch")
const Student = require("./models/Student")
app.post('/admission', async function (req, res) {
    try {
        const batch_id = new mongoose.Types.ObjectId(req.body.batch_id)
        const batchRes = await Batch.aggregate([
            {"$match": {"_id": batch_id},
            {"$lookup": {
                "from": "students",
                "localField": "_id",
                "foreignField": "batchInfo.batch_id",
                "as": "students"
            }}
        ])
        const batch = batchRes[0]
        var studentRolls = []
        for(let i=0; i<batch.students.length; i++) {
            for(let j=0; j<batch.students[i].batchInfo.length; j++) {
                studentRolls.push((Number(batch.students[i].batchInfo[j].studentID)+1).toString().slice(4))
            }
        }
        studentRolls.sort(function(a, b) {return b-a})
        if(studentRolls.length == 0) {
            var studentID = "01"
        } else {
            var studentID = studentRolls[0]
        }
        const name = req.body.name
        const batchInfo = {
            batch_id: batch_id,
            studentID: studentID
        }
        const student = new Student({
            name: name,
            batchInfo: batchInfo
        })
        await student.save()
        res.redirect('/')
    } catch (error) {
        res.send(error)
    }
})

I am using this HTML form (ejs) to create student:

<form method="POST" action="/admission">
    <div>
        <select name="batch_id">
            <option selected>Select Batch</option>
            <% for(let i=0; i<batches.length; i++) { %>
            <option value="<%= batches[i]._id %>"><%= batches[i].academicYear %></option>
            <% } %>
        </select>
        <label>Select Batch</label>
    </div>

    <div class="form-floating mb-3">
        <input type="text" name="name">
        <label>Name</label>
    </div>

    <div>
        <button type="submit">Admit Student</button>
    </div>
</form>

When I create first student, it become succeed. But if I want to create another student, then this error occurs:

{
    "index": 0,
    "code": 11000,
    "keyPattern": {
        "studentID": 1
    },
    "keyValue": {
       "studentID": null
    }
}

I have tried many times, but the error occurs evertime. How to solve this problem?