$ClassMetadata = $em->getClassMetadata($entity_name);
            $column_names = $ClassMetadata->getColumnNames();
            foreach ($ClassMetadata->getAssociationMappings() as $AssociationMapping) {
                if ($AssociationMapping['isOwningSide']) {
                    foreach ($AssociationMapping['joinColumnFieldNames'] as $join_column_field_name) {
                        $column_names[] = $join_column_field_name;
                    }
                }
            }
            $file->fputcsv($column_names);

            $conn = $em->getConnection();
            $sm = $conn->getSchemaManager();
            $columns = $sm->listTableColumns('dtb_product');
            $column_names = array_keys($columns);
            $file->fputcsv($column_names);

            $sql = 'SELECT t.' . implode(',t.', $column_names);
            $sql .= ' FROM ' . $entity_name . ' t';
            $query = $em->createQuery($sql);
            $result = $query->getResult(Query::HYDRATE_ARRAY);
            foreach ($result as $row){
                $file->fputcsv($row);
            }