fill a new row with the given obj

            obj

               instance of the exporter's model

def add_row(self, obj):
        """
        fill a new row with the given obj

            obj

               instance of the exporter's model
        """
        row = {}
        for column in self.headers:

            value = ''

            if '__col__' in column:
                if isinstance(column['__col__'], ColumnProperty):
                    value = self._get_column_cell_val(obj, column)

                elif isinstance(column['__col__'], RelationshipProperty):
                    value = self._get_relationship_cell_val(obj, column)

            row[column['name']] = value

        self._datas.append(self.format_row(row))