Django snippets

from django.db.models.fields import subclassing
from django.core import exceptions
from django.conf import settings
from django.db import models

class TrueNoneField(models.NullBooleanField):
    __metaclass__ = subclassing.SubfieldBase

    def db_type(self):
        if 'postgresql' in settings.DATABASE_ENGINE:
            return "boolean CHECK (%s <> false)" % self.get_attname_column()[1]
        else:
            return super(TrueNoneField, self).db_type()

    def to_python(self, value):
        if value in (True, 1):
            return True
        elif value in (False, 0, None):
            return None

        raise exceptions.ValidationError("This value must be either True or None.")

    def get_db_prep_save(self, value):
        return super(TrueNoneField, self).get_db_prep_save(self.to_python(value))