MDL-31095 quiz editing: only set quiz->grade to 0 if really necessary.

It is only necessary when there are (non-preview) attempts. However,
after making this change we have to ensure users cannot start attempts
when ->grade > 0 and ->sumgrades = 0.

Display ->grade on the order and paging tab of edit.php, so it is clear
what is going on when you are using that tab.

Finally, improve the error message a student gets if they try to start a
quiz with no questions.

mod/quiz/edit.php

@@ -275,8 +275,8 @@ function module_specific_controls($totalnumber, $recurse, $category, $cmid, $cmo
 $remove = optional_param('remove', false, PARAM_INT);
 if (($remove = optional_param('remove', false, PARAM_INT)) && confirm_sesskey()) {
     quiz_remove_question($quiz, $remove);
-    quiz_update_sumgrades($quiz);
     quiz_delete_previews($quiz);
+    quiz_update_sumgrades($quiz);
     redirect($afteractionurl);
 }
 
@@ -496,9 +496,7 @@ function module_specific_controls($totalnumber, $recurse, $category, $cmid, $cmo
 quiz_print_status_bar($quiz);
 
 $tabindex = 0;
-if (!$quiz_reordertool) {
-    quiz_print_grading_form($quiz, $thispageurl, $tabindex);
-}
+quiz_print_grading_form($quiz, $thispageurl, $tabindex);
 
 $notifystrings = array();
 if ($quizhasattempts) {

mod/quiz/editlib.php

@@ -248,8 +248,8 @@ function quiz_add_page_break_after($layout, $questionid) {
 function quiz_save_new_layout($quiz) {
     global $DB;
     $DB->set_field('quiz', 'questions', $quiz->questions, array('id' => $quiz->id));
-    quiz_update_sumgrades($quiz);
     quiz_delete_previews($quiz);
+    quiz_update_sumgrades($quiz);
 }
 
 /**

mod/quiz/lang/en/quiz.php

@@ -127,7 +127,9 @@
 $string['cannotsavequestion'] = 'Cannot save question list';
 $string['cannotsetgrade'] = 'Could not set a new maximum grade for the quiz';
 $string['cannotsetsumgrades'] = 'Failed to set sumgrades';
+$string['cannotstartgradesmismatch'] = 'Cannot start an attempt at this quiz. The quiz is supposed to be graded, but there are no questions in the quiz that are worth any marks.';
 $string['cannotstartmissingquestion'] = 'Cannot start an attempt at this quiz. The quiz definition includes a question that does not exist.';
+$string['cannotstartnoquestions'] = 'Cannot start an attempt at this quiz. The quiz has not been set up yet. No questions have been added.';
 $string['cannotwrite'] = 'Cannot write to export file ({$a})';
 $string['caseno'] = 'No, case is unimportant';
 $string['casesensitive'] = 'Case sensitivity';

mod/quiz/locallib.php

@@ -69,6 +69,11 @@
 function quiz_create_attempt($quiz, $attemptnumber, $lastattempt, $timenow, $ispreview = false) {
     global $USER;
 
+    if ($quiz->sumgrades < 0.000005 && $quiz->grade > 0.000005) {
+        throw new moodle_exception('cannotstartgradesmismatch', 'quiz',
+                new moodle_url('/mod/quiz/view.php', array('q' => $quiz->id)));
+    }
+
     if ($attemptnumber == 1 || !$quiz->attemptonlast) {
         // We are not building on last attempt so create a new attempt.
         $attempt = new stdClass();
@@ -386,10 +391,13 @@ function quiz_has_feedback($quiz) {
  * the grading structure of the quiz is changed. For example if a question is
  * added or removed, or a question weight is changed.
  *
+ * You should call {@link quiz_delete_previews()} before you call this function.
+ *
  * @param object $quiz a quiz.
  */
 function quiz_update_sumgrades($quiz) {
     global $DB;
+
     $sql = 'UPDATE {quiz}
             SET sumgrades = COALESCE((
                 SELECT SUM(grade)
@@ -399,13 +407,20 @@ function quiz_update_sumgrades($quiz) {
             WHERE id = ?';
     $DB->execute($sql, array($quiz->id));
     $quiz->sumgrades = $DB->get_field('quiz', 'sumgrades', array('id' => $quiz->id));
-    if ($quiz->sumgrades < 0.000005 && quiz_clean_layout($quiz->questions, true)) {
-        // If there is at least one question in the quiz, and the sumgrades has been
-        // set to 0, then also set the maximum possible grade to 0.
+
+    if ($quiz->sumgrades < 0.000005 && quiz_has_attempts($quiz->id)) {
+        // If the quiz has been attempted, and the sumgrades has been
+        // set to 0, then we must also set the maximum possible grade to 0, or
+        // we will get a divide by zero error.
         quiz_set_grade(0, $quiz);
     }
 }
 
+/**
+ * Update the sumgrades field of the attempts at a quiz.
+ *
+ * @param object $quiz a quiz.
+ */
 function quiz_update_all_attempt_sumgrades($quiz) {
     global $DB;
     $dm = new question_engine_data_mapper();

mod/quiz/startattempt.php

@@ -50,9 +50,13 @@
 require_login($quizobj->get_courseid(), false, $quizobj->get_cm());
 require_sesskey();
 
-// if no questions have been set up yet redirect to edit.php
-if (!$quizobj->has_questions() && $quizobj->has_capability('mod/quiz:manage')) {
-    redirect($quizobj->edit_url());
+// if no questions have been set up yet redirect to edit.php or display an error.
+if (!$quizobj->has_questions()) {
+    if ($quizobj->has_capability('mod/quiz:manage')) {
+        redirect($quizobj->edit_url());
+    } else {
+        print_error('cannotstartnoquestions', 'quiz', $quizobj->view_url());
+    }
 }
 
 // Create an object to manage all the other (non-roles) access rules.