django admin widget.attrs not work with ForeignKey

[Hassan Igbaria]

self.fields['parent'].widget.attrs = {'class': 'custom-class'}

parent is ForeignKey field

class Category(models.Model):
    title = models.CharField(_('Title'), max_length=300)
    description = models.TextField(_('Description'), null=True, blank=True)
    is_active = models.BooleanField(_('Active ?'), default=True)
    image = models.ImageField(
        _('Image'), upload_to="category/", null=True, blank=True
        )
    parent = models.ForeignKey(
        "self", verbose_name=_('Parent'), on_delete=models.SET_NULL,
        null=True, blank=True
        )