import datetime

from django.db import models
from django.utils import timezone
from django.conf import settings


class DateTimePGUTCField(models.DateTimeField):
    """
    PostgreSQL has a native timestamp type that stores UTC datetime but witout timezone info,
    so we need to convert it in Python.
    """

    def db_type(self, connection):
        if connection.settings_dict['ENGINE'] == 'django.db.backends.postgresql':
            return 'timestamp'
        raise TypeError('this field only works with PostgreSQL')

    def from_db_value(self, value, expression, connection):
        if value is None:
            return value
        if isinstance(value, datetime.datetime):
            if settings.USE_TZ and timezone.is_naive(value):
                return value.replace(tzinfo=datetime.timezone(datetime.timedelta(hours=0)))
            return value
        return super(DateTimePGUTCField, self).to_python(value)