old_contents: from django.db import models
  import uuid
  
  class Flat(models.Model):
      id = models.UUIDField(
          primary_key=True,
          default=uuid.uuid4,
          editable=False,
      )
      complete = models.ForeignKey(
          'Complete',
          related_name='flats',
          on_delete=models.CASCADE,
      )
      selection = models.ForeignKey(
          'Selection',
          related_name='flats',
          on_delete=models.CASCADE,
      )
      score = models.OneToOneField(
          'Score',
          null=True,
          blank=True,
          on_delete=models.SET_NULL,
      )
      class Meta:
          unique_together = (
              ('complete', 'selection', 'score'),
          )
  
new_contents: from django.db import models
  import uuid
  from django.core.exceptions import ValidationError
  
  class Flat(models.Model):
      id = models.UUIDField(
          primary_key=True,
          default=uuid.uuid4,
          editable=False,
      )
      complete = models.ForeignKey(
          'Complete',
          related_name='flats',
          on_delete=models.CASCADE,
      )
      selection = models.ForeignKey(
          'Selection',
          related_name='flats',
          on_delete=models.CASCADE,
      )
      score = models.OneToOneField(
          'Score',
          null=True,
          blank=True,
          on_delete=models.SET_NULL,
      )
      class Meta:
          unique_together = (
              ('complete', 'selection', 'score'),
          )
  
      def clean(self):
          if self.complete.panelist.round != self.selection.song.appearance.round:
              raise ValidationError('Rounds do not match')
  
subject: Add round validator