from django.core import exceptions
from django.db import models


class NullTrueField(models.NullBooleanField):
  """Allow only True or NULL (silently coerce False -> NULL).

  This field type is useful when you have a field that you only want to allow 1
  of.  For example, a "primary" email address.

      class Email(models.Model):
        user = models.ForeignKey('auth.User')
        primary = NullTrueField()

        class Meta:
          unique_together = ('user', 'primary')

  By adding the ``unique_together`` constraint, we can enforce a single primary
  email address per user (since NULL != NULL).
  """
  description = "Boolean, Either True or None (no False allowed)."
  __metaclass__ = models.SubfieldBase

  def to_python(self, value):
    """Handle True/None normally, silently coerce False to None"""
    if value is None:
      return None
    if value is True:
      return True
    if value in ('None',):
      return None
    if value in ('t', 'True', '1'):
      return True

    # Coerce False -> None
    if value is False:
      return None
    if value in ('f', 'False', '0'):
      return None

    raise exceptions.ValidationError(
      self.error_messages['invalid'],
      code='invalid',
      params={'value': value},
    )

  def get_prep_value(self, value):
    value = super(NullTrueField, self).get_prep_value(value)
    if bool(value):
      return True
    return None