clean-unique-field.py

from django import forms
from django.core.exceptions import ValidationError

def clean_unique(form, field, exclude_initial=True, 
                 format="The %(field)s %(value)s has already been taken."):
    value = form.cleaned_data.get(field)
    if value:
        qs = form._meta.model._default_manager.filter(**{field:value})
        if exclude_initial and form.initial:
            initial_value = form.initial.get(field)
            qs = qs.exclude(**{field:initial_value})
        if qs.count() > 0:
            raise forms.ValidationError(format % {'field':field, 'value':value})
    return value

# Usage:

class DeployForm(forms.ModelForm):
    """We want both the slug and cname fields to be unique"""

    class Meta:
        model = Website
        fields = ['slug', 'cname']
        
    def clean_slug(self):
        return clean_unique(self, 'slug')
        
    def clean_cname(self):
        return clean_unique(self, 'cname')