Custom API method for export report by ID

I’m trying to add a method to export a report by id.
has many reports.
created an API method
API return JSON = OK.

but
calling any sql always returns the same result:

  "current_field": null,
  "field_count": null,
  "lengths": null,
  "num_rows": null,
  "type": null

why?

and

$module->db->database =   {
        "affected_rows": null,
        "client_info": null,
        "client_version": null,
        "connect_errno": null,
        "connect_error": null,
        "errno": null,
        "error": null,
        "error_list": null,
        "field_count": null,
        "host_info": null,
        "info": null,
        "insert_id": null,
        "server_info": null,
        "server_version": null,
        "stat": null,
        "sqlstate": null,
        "protocol_version": null,
        "thread_id": null,
        "warning_count": null
}

only null

try code:
\service\v4\SugarWebServiceImplv4.php

    public function get_report()
    {
        include_once "modules/AOR_Reports/AOR_Report.php";

        $report = new AOR_Report();
        $report->report_module = 'AOR_Reports';
        $report->id = $_REQUEST['report_id'];

        $report->build_api_report();
    }

\modules\AOR_Reports\AOR_Report.php

    public function build_api_report()
    {
        $this->build_report_csv(); // default CSV export method
    }

result: return only csv headers