Django model constraint or business logic - python-3.x

Question

I am building my first django project, it will essentially be an assessment form. Each question will be multiple choice. There is no right or wrong answer the questions but instead a grade of what level they are at. So for each question a user can only select one answer out of the possible three options.
I have defined the following models
from django.db import models
# Create your models here.
class Category(models.Model):
name = models.CharField(max_length=100)
def __str__(self):
return self.name
class Meta:
verbose_name_plural = 'Categories'
class Question(models.Model):
category = models.ForeignKey(Category, on_delete=models.PROTECT)
capability = models.TextField()
weight = models.FloatField(default=1.0)
def __str__(self):
return self.capability
class Grade(models.Model):
name = models.CharField(max_length=100)
score = models.PositiveIntegerField()
def __str__(self):
return self.name
class Answer(models.Model):
question = models.ForeignKey(Question, on_delete=models.PROTECT)
grade = models.ForeignKey(Grade, on_delete=models.PROTECT)
description = models.TextField()
def __str__(self):
return f'Q{self.question.id} - {self.description}'
class Area(models.Model):
name = models.CharField(max_length=100)
def __str__(self):
return self.name
class Assessment(models.Model):
created = models.DateTimeField(auto_now_add=True)
area = models.ForeignKey(Area, on_delete=models.PROTECT)
answers = models.ManyToManyField(Answer)
def __str__(self):
return f'{self.area} - {self.created}'
While an assessment will have multiple answers, it should only be able to contain one answer per question. Can this type of constraint be designed in the model? or it would be part of the business logic. Esentially the user will pick an answer in the webpage from a radio input so will only be able to select one. The admin page will allow me to select multiple answers form the same question.
I am just trying to find out if there is an elegant way to design this within the model, or just use the business logic to ensure only one answer per question is allowed.