allow deletion of quizzes with student submissions

fixes CNVS-9050

test plan:
- create a quiz
- take the quiz as a student
- as a teacher try to delete the quiz
- the quiz should successfully delete

Change-Id: Id53a0482217d1a185d15d60358e8bd41680b5a6b
Reviewed-on: https://gerrit.instructure.com/25546
Reviewed-by: Stanley Stuart <stanley@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>

Author: Derek DeVries

app/models/quiz.rb

@@ -310,7 +310,7 @@ def update_assignment
         a.assignment_group_id = self.assignment_group_id
         a.saved_by = :quiz
         a.workflow_state = 'published' if a.deleted?
-        if context.draft_state_enabled?
+        if context.draft_state_enabled? && !deleted?
           a.workflow_state = self.published? ? 'published' : 'unpublished'
         end
         a.notify_of_update = @notify_of_update

spec/models/quiz_spec.rb

@@ -1098,6 +1098,31 @@
     end
   end
 
+  context "#destroy" do
+    it "should logical delete published quiz" do
+      quiz = @course.quizzes.create(title: 'test quiz')
+      quiz.context.root_account.enable_draft!
+      quiz.stubs(:has_student_submissions? => true)
+      quiz.publish!
+      quiz.assignment.stubs(:has_student_submissions? => true)
+
+      quiz.destroy
+      quiz.deleted?.should be_true
+    end
+
+    it "should logical delete the published quiz's associated assignment" do
+      quiz = @course.quizzes.create(title: 'test quiz')
+      quiz.context.root_account.enable_draft!
+      quiz.stubs(:has_student_submissions?).returns true
+      quiz.publish!
+      assignment = quiz.assignment
+      assignment.stubs(:has_student_submissions?).returns true
+
+      quiz.destroy
+      assignment.deleted?.should be_true
+    end
+  end
+
   context "draft_state" do
 
     it "updates the assignment's workflow state" do