Currently only checks the field's uniqueness, not the model validation.

def _val_is_unique(val, field):
        """
        Currently only checks the field's uniqueness, not the model validation.
        """
        if val is None:
            return False

        if not field.unique:
            return True

        field_name = field.name
        return field.model.objects.filter(**{field_name: val}).count() == 0