Limit number of foreign keys

Question

Let's take this example:

class Team (models.Model):
    name = models.CharField('Name', max_length=30)

class Round (models.Model):
    round_number = models.IntegerField('Round', editable=False) #Auto-incrementing per Team
    team = models.ForeignKey(Team)

There is a limit of 3 rounds. How can I raise an error inside the Admin and generally prevent a team from having more than 3 Rounds?

Answer 1

I liked the way using the validator:

def restrict_amount(value):
    if Round.objects.filter(team_id=value).count() >= 3:
        raise ValidationError('Team already has maximal amount of rounds (3)')


class Team (models.Model):
    name = models.CharField('Name', max_length=30)


class Round (models.Model):
    round_number = models.IntegerField('Round', editable=False) #Auto-incrementing per Team
    team = models.ForeignKey(Team, validators=(restrict_amount, ))

Using validator will make Django handle it properly, for example, display an error in the admin panel.