Rest django checking the duplicate number - django

Question

if i inserting the number 5 i want to check all the numbers before 5( like 1,2,3 and 4 )is must have inside the db ,then only can i add the number 5 ,otherwise show error. And also check is any duplication in the Db using rest django model serilaizer.
def validate_match_round(self, match_round):
if not match_round :
raise serializers.ValidationError("Enter value more than 0 ")
matchscore model
class Matchscore(TimeStampedModel):
gameevent = models.ForeignKey(GameEvent, null=True, related_name='game_event',on_delete=models.DO_NOTHING)
match_round = models.IntegerField(null=True,blank=True)
team_a = models.ForeignKey(Team,null=True,related_name='team_one',on_delete=models.DO_NOTHING)
team_a_score = models.PositiveIntegerField(null=True,blank=True)
team_b = models.ForeignKey(Team,null=True,related_name='team_two',on_delete=models.DO_NOTHING)
team_b_score = models.PositiveIntegerField(null=True,blank=True)
team_won = models.ForeignKey(Team,null=True,related_name='team', on_delete=models.DO_NOTHING)
if match_round == 1:
return match_round
match = Matchscore.objects.aggregate(Max('match_round'))
print(match)
if match_round == match["match_round__max"] + 1 :
return match_round
else:
raise serializers.ValidationError("Enter a valid match_round")

Answer 1

all_rounds = Matchscore.objects.filter(match_round__lte=match_round).order_by('match_round').values_list('match_round', flat=True)
Will return list of match_round values that lower or equal than match_round passed to function
all_rounds_count = Matchscore.objects.filter(match_round__lte=match_round).count()
Will return count of these rounds. and if (all_rounds_count + 1) == match_round check is what you want
And also check is any duplication in the Db using rest django model serilaizer.
To avoid keys duplication
Set unique=True for model.Field to disable round duplicates match_round = models.IntegerField(..., unique = True)
Add unique validator to serializer