def _delete_null_columns(self):
        """
        executes a delete query to remove columns that have changed to null
        """
        ds = DeleteStatement(self.column_family_name)
        deleted_fields = False
        for _, v in self.instance._values.items():
            col = v.column
            if v.deleted:
                ds.add_field(col.db_field_name)
                deleted_fields = True
            elif isinstance(col, Map):
                uc = MapDeleteClause(col.db_field_name, v.value, v.previous_value)
                if uc.get_context_size() > 0:
                    ds.add_field(uc)
                    deleted_fields = True

        if deleted_fields:
            for name, col in self.model._primary_keys.items():
                ds.add_where_clause(WhereClause(
                    col.db_field_name,
                    EqualsOperator(),
                    col.to_database(getattr(self.instance, name))
                ))
            self._execute(ds)
executes a delete query to remove columns that have changed to null