old_contents: from django.db import models
  from rest_framework import serializers
  
  class Question(models.Model):
      version = models.CharField(primary_key=True, max_length=8)
      text = models.TextField()
      created_on = models.DateTimeField(auto_now_add=True)
      updated_on = models.DateTimeField(auto_now=True)
  
  class Choice(models.Model):
      text = models.TextField()
      version = models.ForeignKey(Question, on_delete=models.CASCADE)
      created_on = models.DateTimeField(auto_now_add=True)
      updated_on = models.DateTimeField(auto_now=True)
  
  class Answer(models.Model):
      choice = models.ForeignKey(Choice, on_delete=models.CASCADE)
      user_id = models.TextField()
      created_on = models.DateTimeField(auto_now_add=True)
  
  class ChoiceSerializer(serializers.ModelSerializer):
      class Meta:
          model = Choice
          fields = ('id', 'text', 'version', 'created_on', 'updated_on',)
  
  class QuestionSerializer(serializers.ModelSerializer):
      choices = ChoiceSerializer(many=True)
      class Meta:
          model = Question
          fields = ('text', 'version', 'choices', 'created_on', 'updated_on',)
  
  class AnswerSerializer(serializers.ModelSerializer):
      class Meta:
          model = Answer
          fields = ('id', 'choice_id', 'user_id', 'created_on',)
  
new_contents: from django.db import models
  from rest_framework import serializers
  
  class Question(models.Model):
      version = models.CharField(primary_key=True, max_length=8)
      text = models.TextField()
      created_on = models.DateTimeField(auto_now_add=True)
      updated_on = models.DateTimeField(auto_now=True)
  
  class Choice(models.Model):
      text = models.TextField()
      version = models.ForeignKey(Question, on_delete=models.CASCADE)
      created_on = models.DateTimeField(auto_now_add=True)
      updated_on = models.DateTimeField(auto_now=True)
  
  class Answer(models.Model):
      choice = models.ForeignKey(Choice, on_delete=models.CASCADE)
      user_id = models.TextField()
      created_on = models.DateTimeField(auto_now_add=True)
  
  class ChoiceSerializer(serializers.ModelSerializer):
      class Meta:
          model = Choice
          fields = ('id', 'text', 'version', 'created_on', 'updated_on',)
  
  class QuestionSerializer(serializers.ModelSerializer):
      # TODO: create a serializer that returns list of choices for the question
      class Meta:
          model = Question
          fields = ('text', 'version', 'created_on', 'updated_on',)
  
  class AnswerSerializer(serializers.ModelSerializer):
      class Meta:
          model = Answer
          fields = ('id', 'choice_id', 'user_id', 'created_on',)
  
subject: Refactor QuestionSerializer