Use the value captured from URL with DRF Serializer

  from django.db import models


  class Todo(models.Model):
      headline = models.CharField(max_length=200)
      content = models.TextField()
      created_at = models.DateTimeField(auto_now_add=True)


  class Comment(models.Model):
      todo = models.ForeignKey(Todo, on_delete=models.CASCADE)
      title = models.CharField(max_length=200)
      body = models.TextField()
      created_at = models.DateTimeField(auto_now_add=True)

      class Meta:
          unique_together = (('todo', 'title'),)

The parent Todo id of the Comment is specified in the URL.

  router.register(
      r'todos/(?P<todo_id>\d)/comments',
      CommentViewSet,
      basename='todo_comment',
  )

Pass to Serializer via get_serializer_context of ViewSet.

  def get_serializer_context(self):
      context = super().get_serializer_context()
      context['todo_id'] = self.kwargs['todo_id']
      return context

  def to_internal_value(self, data):
      values = super().to_internal_value(data)
      values['todo_id'] = self.context['todo_id']
      return values

I feel right as a method usage, todo_id validation is omitted for simplicity.

• https://www.django-rest-framework.org/api-guide/serializers/#read-write-baseserializer-classes

  This method returns the validated values that will be used to construct the object instance, and may raise a serializers.ValidationError if the supplied data is in an incorrect format.