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


class NullCharField(models.CharField):
    """
    CharField that stores '' as NULL and returns NULL as ''.
    Use this for unique CharField.

    As long as this field is referenced in a migration, this definition must be
    kept.

    When upgrading to Django >= 1.11, it is possible to drop the methods:
        `from_db_value`, `to_python` and `get_prep_value`.

    Source:
        https://github.com/django-oscar/django-oscar/blob/1a2e8279161a396e70e44339d7206f25355b33a3/src/oscar/models/fields/__init__.py#L94
    """

    description = "CharField that stores '' as NULL and returns NULL as ''"

    def __init__(self, *args, **kwargs):
        if not kwargs.get("null", True) or not kwargs.get("blank", True):
            raise ImproperlyConfigured("NullCharField implies 'null == blank == True'.")
        kwargs["null"] = kwargs["blank"] = True
        super().__init__(*args, **kwargs)

    def from_db_value(self, value, *args, **kwargs):
        return self.to_python(value)

    def to_python(self, *args, **kwargs):
        val = super().to_python(*args, **kwargs)
        return val if val is not None else ""

    def get_prep_value(self, *args, **kwargs):
        prepped = super().get_prep_value(*args, **kwargs)
        return prepped if prepped != "" else None

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        del kwargs["null"]
        del kwargs["blank"]
        return name, path, args, kwargs