When deleting questions Moodle tries to detect used ones but only query a table from the QUIZ module. This breaks the plug-ability of this part of Moodle.

Problematic code (editlib.php):

    if (record_exists('quiz_question_instances', 'question', $key)) {
      $questionnames .= '* ';
      $inuse = true;
    }

We propose this set of changes to solve the issue:

Change above lines to be:

    $inuse = question_is_question_in_use($key);

And define the new function:

function question_is_question_in_use($questionid)
{
  global $CFG;
  require_once($CFG->libdir . '/ddllib.php');
  $modules = get_records("modules");
// Scan all modules looking for a table named modulename_question_instances
  $question_in_use = false;
  foreach ($modules as $module)
  {
    $table = new XMLDBTable($module->name.'_question_instances');
    if (table_exists($table))
    {
      if (record_exists($module->name.'_question_instances', 'question', $questionid))
      {
return true;
      }
    }
  }
  return false;
}

This way default functionality remains the same and new modules can register the use of question instances by defining a table named "mdlprefix_modulename_question_instances" (like QUIZ does).

The same piece of code is reused in the function question_showbank_actions:

            if (record_exists('quiz_question_instances', 'question', $questionid)) {
              if (!set_field('question', 'hidden', 1, 'id', $questionid)) {
                question_require_capability_on($questionid, 'edit');
                error('Was not able to hide question');
              }
            } else {
              delete_question($questionid);
            }

We propose to change it to:

         question_delete_or_hide($questionid);


Being this function:


function question_delete_or_hide($questionid)
{
  $question_in_use = question_is_question_in_use($questionid);
 
// If the question is instanced in any module do not delete it, just hide it.
  if ($question_in_use == true)
  {
    if (!set_field('question', 'hidden', 1, 'id', $questionid))
    {
      question_require_capability_on($questionid, 'edit');
      error('Was not able to hide question');
    }
  }
  else
  {
    delete_question($questionid);
  }
}

This approach has been tested and we don't see any adverse side-effects.