Fix #114 - when downloading, return empty csv file (with headers) whe…

…n no results

lib.php

@@ -89,7 +89,7 @@ function report_customsql_pluginfile($course, $cm, $context, $filearea, $args, $
         if ($report->runable !== 'manual') {
             $runtime = $report->lastrun;
         }
-        $csvtimestamp = report_customsql_generate_csv($report, $runtime);
+        $csvtimestamp = \report_customsql_generate_csv($report, $runtime, true);
     }
     list($csvfilename) = report_customsql_csv_filename($report, $csvtimestamp);
 

locallib.php

@@ -96,14 +96,29 @@ function report_customsql_get_element_type($name) {
     return 'text';
 }
 
-function report_customsql_generate_csv($report, $timenow) {
+/**
+ * Generate customsql csv file.
+ *
+ * @param stdclass $report report record from customsql table.
+ * @param int $timetimenow unix timestamp - usually "now()"
+ * @param bool $returnheaderwhenempty if true, a CSV file with headers will always be generated, even if there are no results.
+ */
+function report_customsql_generate_csv($report, $timenow, $returnheaderwhenempty = false) {
     global $DB;
     $starttime = microtime(true);
 
     $sql = report_customsql_prepare_sql($report, $timenow);
 
     $queryparams = !empty($report->queryparams) ? unserialize($report->queryparams) : [];
     $querylimit  = $report->querylimit ?? get_config('report_customsql', 'querylimitdefault');
+    if ($returnheaderwhenempty) {
+        // We want the export to always generate a CSV file so we modify the query slightly
+        // to generate an extra "null" values row, so we can get the column names,
+        // then we ignore rows that contain null records in every row when generating the csv.
+        $sql = "SELECT subq.*
+                  FROM (SELECT 1) as ignoreme
+                  LEFT JOIN ($sql) as subq on true";
+    }
     // Query one extra row, so we can tell if we hit the limit.
     $rs = report_customsql_execute_query($sql, $queryparams, $querylimit + 1);
 
@@ -124,6 +139,11 @@ function report_customsql_generate_csv($report, $timenow) {
         }
 
         $data = get_object_vars($row);
+
+        if ($returnheaderwhenempty && array_unique(array_values($data)) === [null]) {
+            // This is a row with all null values - ignore it.
+            continue;
+        }
         foreach ($data as $name => $value) {
             if (report_customsql_get_element_type($name) == 'date_time_selector' &&
                     report_customsql_is_integer($value) && $value > 0) {

tests/behat/behat_report_customsql.php

@@ -248,6 +248,28 @@ public function adhoc_database_queries_thinks_the_time_is($time) {
         set_config('behat_fixed_time', $value, 'report_customsql');
     }
 
+    /**
+     * Simulates downloading an empty report to ensure it shows table headers.
+     *
+     * For example:
+     * When downloading the empty custom sql report "Frog" it contains the headers "frogname,freddy"
+     *
+     * @Then /^downloading custom sql report "(?P<REPORT_NAME>[^"]*)" returns a file with headers "([^"]*)"$/
+     * @param string $reportname the name of the report to go to.
+     * @param string $headers the headers that shuold be returned.
+     */
+    public function downloading_custom_sql_report_x_returns_a_file_with_headers(string $reportname, string $headers) {
+        $report = $this->get_report_by_name($reportname);
+        $url = new \moodle_url('/pluginfile.php/1/'.'report_customsql'. '/'.'download'. '/'. $report->id, ['dataformat' => 'csv']);
+
+        $session = $this->getSession()->getCookie('MoodleSession');
+        $filecontent = trim(download_file_content($url, array('Cookie' => 'MoodleSession=' . $session)));
+        $filecontent = core_text::trim_utf8_bom($filecontent);
+        if ($filecontent != $headers) {
+            throw new \Behat\Mink\Exception\ExpectationException("File headers: $filecontent did not match expected: $headers", $this->getSession());
+        }
+    }
+
     /**
      * Find a report by name and get all the details.
      *

tests/behat/report_customsql.feature

@@ -76,6 +76,13 @@ Feature: Ad-hoc database queries report
     And I view the "Test query" custom sql report
     Then I should see "This query did not return any data."
 
+  Scenario: Download an Ad-hoc database query that returns no data but includes headers
+    Given the following custom sql report exists:
+      | name     | Test query                               |
+      | querysql | SELECT * FROM {config} WHERE name = '-1' |
+    When I log in as "admin"
+    Then downloading custom sql report "Test query" returns a file with headers "id,name,value"
+
   Scenario: Create an Ad-hoc database queries category
     When I log in as "admin"
     And I navigate to "Reports > Ad-hoc database queries" in site administration