Set default values to fields. We assume that they are not yet populated
        as this method is called just after creation of a new pk.

def _set_defaults(self):
        """
        Set default values to fields. We assume that they are not yet populated
        as this method is called just after creation of a new pk.
        """
        for field_name in self._fields:
            if field_name in self._init_fields:
                continue
            field = self.get_field(field_name)
            if hasattr(field, "default"):
                field.proxy_set(field.default)