MongoDB schema design for multpile choice questions and answers

Question

I'm not skilled in MongoDB designing and I need help in designing a DB. What is the best structure for Storing Questions having answer choices and Answers from candidate?

-Each candidate will get a set of 12 questions if the candidate fails in the first exam they can attend 2 more exams. So on each exam candidate should get different set of questions each time.

-Answer by each candidate for each test will have to be recorded with the score out of 12 as each question set is 12.

Answer 1

I have created a mongoose schema for each required details. You can take help from it. I have analyzed a bit your requirements and adding models for many schemas, first question schema, exported as a model

import { Schema } from 'mongoose';
import { AnswerOptionSchema } from './answer-option-schema';
const mongoose = require('mongoose');

export const QuestionSchema: Schema = new Schema({
  question: {
    type: String,
    minlength: 10,
    maxlength: 1000,
  },
  answerOptions: {
    type: [AnswerOptionSchema],
    default: undefined,
    validate: {
      validator: function(value: any) {
        return value && value.length === 4;
      },
      message: 'Answer options should be 4.'
    }
  }
}, {
  timestamps: true
});

export const Question = mongoose.model('Question', QuestionSchema);

and here in the QuestionSchema, I have embedded an AnswerOptionSchema as

import { Schema } from 'mongoose';

export const AnswerOptionSchema: Schema = new Schema({
  optionNumber: {
    type: Number
  },
  answerBody: {
    type: String,
    minlength: 1,
    maxlength: 200,
  },
  isCorrectAnswer: { // you can store the correct answer with question id in another model.
    type: Boolean,
    default: false
  }
}, {
  _id: false
});

With the help of these schemas, I have created a QuestionSetSchema to add a set of question schema as

import { Schema } from "mongoose";
import { QuestionSchema } from "./question-schema";
const mongoose = require('mongoose');

export const QuestionSetSchema: Schema = new Schema({
  questionSet: {
    type: [QuestionSchema],
    validate: {
      validator: function(value: any) {
        return value.length === 12;
      },
      message: 'Question set must be 12.'
    }
  },
}, {
  timestamps: true
});

export const QuestionSet = mongoose.model('QuestionSet', QuestionSetSchema);

Now prepared with the question, answer options and the set, now need to design the candidate schema,

import { Schema } from "mongoose";
const mongoose = require('mongoose');

export const CandidateSchema: Schema = new Schema({
  name: String,
  email: String, // you can store other candidate related information here.
  totalAttempt: {
    type: Number,
    default: 0,
    validate: {
      validator: function(value: number) {
        return value === 3;
      },
      message: 'You have already done three attempts.'
    }
  },
  candidateQuestionAnswers: {
    type: [Schema.Types.ObjectId],
    ref: 'CandidateQuesAnswer'
  }
}, {
  timestamps: true
});

export const Candidate = mongoose.model('Candidate', CandidateSchema);

Here, you will notice, I am also calculating the totalAttempt of the candidate and the answers for each set given by him in the CandidateQuesAnswer model. This model has the structure like

import { Schema } from "mongoose";

export const CandidateQuesAnswerSchema = new Schema({
  candidate: {
    type: Schema.Types.ObjectId,
    ref: 'Candidate'
  },
  questionSet: {
    type: Schema.Types.ObjectId,
    ref: 'QuestionSet'
  },
  questionAnswers: {
    type: [Number] // You can add answer schema
  },
  totalScore: {
    type: Number
  },
  isPassed: {
    type: Boolean,
    default: false
  }
}, {
  timestamps: true
});

CandidateQuesAnswerSchema.pre('save', function updateTotalScore(next) {
  // update total score of the candidate here based on the correct questionAnswers and
  // questionSet.
  next();
});

CandidateQuesAnswerSchema.pre('save', function updateIsPassed(next) {
  // update the isPassed based on the totalScore obtained by the candidate.
  next();
});

export const CandidateQuesAnswer = mongoose.model('CandidateAnswer', CandidateQuesAnswerSchema);

Where I have used pre save hooks provided by mongoose, before saving the document and calculating the values to declare the candidate pass or fail.