const mongoose = require('mongoose');
const { Schema } = mongoose;

const Students = new Schema({
  eno: {
    type: number,
    required: true,
    unique: true,
  },
  personal_info: {
    type: Schema.Types.ObjectId,
    required: true,
    ref: 'StdInfo',
  },
  grades: {
    type: Schema.Types.ObjectId,
    required: true,
    ref: 'StdMarks',
  }
});
const StudentsModel = mongoose.model('Students', Students);
...