[Django]-Django – How to specify which field a validation fails on?

class Dog(models.Model):
    bark_volume = models.DecimalField(...
    unladen_speed = models.DecimalField(...

    def clean(self):
        if self.bark_volume < 5:
            if not self._errors.has_key('bark_volume'):
                from django.forms.util import ErrorList
                self._errors['bark_volume'] = ErrorList()
            self._errors['bark_volume'].append('must be louder!')

That works on forms, at least. Never tried it on the model itself, but the methodology should be the same. However, from the Django docs:

When you use a ModelForm, the call to is_valid() will perform these validation steps for all the fields that are included on the form. (See the ModelForm documentation for more information.) You should only need to call a model’s full_clean() method if you plan to handle validation errors yourself, or if you have excluded fields from the ModelForm that require validation.

And…

Note that full_clean() will not be called automatically when you call your model’s save() method, nor as a result of ModelForm validation. You’ll need to call it manually when you want to run model validation outside of a ModelForm.

So, basically, unless you have a really good reason to do field cleaning on the model, you should do it on the form instead. The code for that would look like:

class DogForm(forms.ModelForm):

    def clean(self):
        bark_volume = self.cleaned_data.get('bark_volume')
        if bark_volume < 5:
            if not self._errors.has_key('bark_volume'):
                from django.forms.util import ErrorList
                self._errors['bark_volume'] = ErrorList()
            self._errors['bark_volume'].append('must be louder!')

        return self.cleaned_data

And that will work, for sure.

To note for anyone that may come across this with a newer version of Django – the clean_fields method from the accepted answer now requires an “exclude” param. Also – I believe the accepted answer is also missing a call to it’s super function. The final code that I used was:

def clean_fields(self, exclude=None):
    super(Model, self).clean_fields(exclude)

    if self.field_name and not self.field_name_required:
        raise ValidationError({'field_name_required':["You selected a field, so field_name_required is required"]})

abbreviated, from the django docs:

def clean(self):
    data = self.cleaned_data
    subject = data.get("subject")

    if subject and "help" not in subject:
        msg = "Must put 'help' in subject."
        self.add_error('subject', msg)

    return data

Use a clean_ method that is specific to the field:

class DogForm(forms.ModelForm):
    class Meta:
        model = Dog

    def clean_bark_volume(self):
        if self.cleaned_data['bark_volume'] < 5:
            raise ValidationError("must be louder!")

See the clean<fieldname> part of the Form Validation page. Also, make sure to use cleaned_data instead of the form field itself; the latter may have old data. Finally, do this on the form and not the model.

The simplest way to validate this particular case would be:

from django.core.validators import MinValueValidator
from django.utils.translation import ugettext_lazy as _

class Dog(models.Model):
    bark_volume = models.DecimalField(
        ..., validators=[MinValueValidator(5, message=_("Must be louder!"))]

Django’s documentation about validators: https://docs.djangoproject.com/en/dev/ref/validators/