Django model validation not raising Exception on full_clean()

March 7, 2022

I have a Model and ModelForm with custom validator (which only allows "H" or "A" in the CharField):

def home_away_valid(value):
    return value == 'H' or value == 'A'


class Team(models.Model):
    name = models.CharField(max_length=180)
    home = models.CharField(max_length=2, validators=[home_away_valid], default='H', db_index=True)


class TeamForm(ModelForm):
    class Meta:
        model = Team
        fields = ['home', 'name']

However when I run full_clean() with another value (not H or A) it doesn’t not raise a validation exception:

try:
    team = TeamForm({
        'name': 'Test Team',
        'home': 'S'
    })
    team.full_clean()
    new_team = team.save()
    print(new_team.id)
except ValidationError as e:
    print(e)

Why does this not raise an Exception? (I have tried doing the full_clean() on both the ModelForm and Model, neither raises an Exception)

>Solution :

A validator should raise a ValidationError in case the condition is not met, not return True or False, so:

from django.core.exceptions import ValidationError

def home_away_valid(value):
    if value not in ('H', 'A'):
        raise ValidationError('Must be home or away')

You also might want to use 'H' and 'A' as choices, this will render the form with a ChoiceField, making it less likely to make mistakes:

class Team(models.Model):
    HOME_AWAY = (
        ('H', 'Home'),
        ('A', 'Away')
    )

    name = models.CharField(max_length=180)
    home = models.CharField(max_length=2, choices=HOME_AWAY, validators=[home_away_valid], default='H', db_index=True)