fix changing quiz with overrides to ungraded

test plan:
 - create a graded quiz
 - set a due date for it for a specific student
 - change the quiz to ungraded
 - the quiz should save successfully
 - the override should still appear on the quiz page
   and the quiz edit page
 - you should be able to remove the override and later
   assign the student to a different override

fixes ADMIN-1058

Change-Id: Ia8f0d3d581ab897046ba91b692d9cbcc1639d0ed
Reviewed-on: https://gerrit.instructure.com/150912
Tested-by: Jenkins
Reviewed-by: Mysti Sadler <mysti@instructure.com>
Product-Review: Mysti Sadler <mysti@instructure.com>
QA-Review: Mysti Sadler <mysti@instructure.com>

app/controllers/quizzes/quizzes_controller.rb

@@ -475,7 +475,10 @@ class Quizzes::QuizzesController < ApplicationController
               @quiz.assignment.save
             end
 
-            perform_batch_update_assignment_overrides(@quiz, prepared_batch) unless overrides.nil?
+            unless overrides.nil?
+              update_quiz_and_assignment_versions(@quiz, prepared_batch) # to prevent undoing Quiz#link_assignment_overrides
+              perform_batch_update_assignment_overrides(@quiz, prepared_batch)
+            end
 
             # quiz.rb restricts all assignment broadcasts if notify_of_update is
             # false, so we do the same here
@@ -494,7 +497,7 @@ class Quizzes::QuizzesController < ApplicationController
           end
         end
 
-        if @overrides_affected.to_i > 0 || cached_due_dates_changed
+        if @quiz.assignment && (@overrides_affected.to_i > 0 || cached_due_dates_changed)
           DueDateCacher.recompute(@quiz.assignment, update_grades: true)
         end
 
@@ -1005,4 +1008,11 @@ class Quizzes::QuizzesController < ApplicationController
   def get_quiz_params
     params[:quiz] ? params[:quiz].permit(API_ALLOWED_QUIZ_INPUT_FIELDS[:only]) : {}
   end
+
+  def update_quiz_and_assignment_versions(quiz, prepared_batch)
+    params = { quiz_id: quiz.id, quiz_version: quiz.version_number,
+               assignment_id: quiz.assignment_id, assignment_version: quiz.assignment&.version_number }
+    prepared_batch[:overrides_to_create].each { |override| override.assign_attributes(params) }
+    prepared_batch[:overrides_to_update].each { |override| override.assign_attributes(params) }
+  end
 end

spec/controllers/quizzes/quizzes_controller_spec.rb

@@ -1466,6 +1466,28 @@ describe Quizzes::QuizzesController do
       expect(overrides.length).to eq 1
     end
 
+    it "can change a graded quiz with overrides into an ungraded quiz" do
+      user_session(@teacher)
+      quiz = @course.quizzes.create!(:title => 'blah', :quiz_type => 'assignment')
+      override = create_adhoc_override_for_assignment(quiz, @student)
+      post 'update', :params => {
+        :course_id => @course.id,
+        :id => quiz.id,
+        :quiz => {
+          :quiz_type => 'survey',
+          :assignment_overrides => [{
+            :id => override.id,
+            :assignment_id => quiz.assignment.id,
+            :title => '1 student',
+            :student_ids => [@student.id]
+          }]
+        }
+      }
+      expect(quiz.reload.assignment_id).to be_nil
+      expect(override.reload.assignment_id).to be_nil
+      expect(override.quiz_id).to eq quiz.id
+    end
+
     describe "DueDateCacher" do
       before :each do
         user_session(@teacher)