MDL-56646 assign: Don't rescale any negative grades

mod/assign/lib.php

@@ -1585,7 +1585,8 @@ function assign_rescale_activity_grades($course, $cm, $oldmin, $oldmax, $newmin,
         'a' => $cm->instance
     );
 
-    $sql = 'UPDATE {assign_grades} set grade = (((grade - :p1) * :p2) + :p3) where assignment = :a';
+    // Only rescale grades that are greater than or equal to 0. Anything else is a special value.
+    $sql = 'UPDATE {assign_grades} set grade = (((grade - :p1) * :p2) + :p3) where assignment = :a and grade >= 0';
     $dbupdate = $DB->execute($sql, $params);
     if (!$dbupdate) {
         return false;

mod/assign/tests/lib_test.php

@@ -655,4 +655,33 @@ public function test_mod_assign_completion_get_active_rule_descriptions() {
         $this->assertEquals(mod_assign_get_completion_active_rule_descriptions($moddefaults), $activeruledescriptions);
         $this->assertEquals(mod_assign_get_completion_active_rule_descriptions(new stdClass()), []);
     }
+
+    /**
+     * Test that if some grades are not set, they are left alone and not rescaled
+     */
+    public function test_assign_rescale_activity_grades_some_unset() {
+        $this->resetAfterTest();
+
+        // As a teacher...
+        $this->setUser($this->editingteachers[0]);
+        $assign = $this->create_instance();
+
+        // Grade the student.
+        $data = ['grade' => 50];
+        $assign->testable_apply_grade_to_user((object)$data, $this->students[0]->id, 0);
+
+        // Try getting another students grade. This will give a grade of -1.
+        $assign->get_user_grade($this->students[1]->id, true);
+
+        // Rescale.
+        assign_rescale_activity_grades($this->course, $assign->get_course_module(), 0, 100, 0, 10);
+
+        // Get the grades for both students.
+        $student0grade = $assign->get_user_grade($this->students[0]->id, true);
+        $student1grade = $assign->get_user_grade($this->students[1]->id, true);
+
+        // Make sure the real grade is scaled, but the -1 stays the same.
+        $this->assertEquals($student0grade->grade, 5);
+        $this->assertEquals($student1grade->grade, -1);
+    }
 }