Fix module cache on discussion section change

Test Plan:
  - Change `config.action_controller.perform_caching` from false to true
    in config/environments/development.rb
  - Create a course with two sections
  - Create a student in the course that belongs to one of the sections
  - Create a non-graded discussion in the course, that can be viewed by
    all sections
  - Create a module in the course, then add the discussion to that
    module
  - As the student, visit the module, and verify you can see the
    discussion
  - As a teacher, modify the discussion so that it is restricted to the
    section that the student is not in
  - Refresh the module page as the student, and notice that you no
    longer see the discussion
  - Change `config.action_controller.perform_caching` in step 1 back to
    false

fixes COMMS-2045

flag = none

Change-Id: I230444533c3f446cf996a632275f60f3bab301a8
Reviewed-on: https://gerrit.instructure.com/211575
Tested-by: Jenkins
QA-Review: Landon Gilbert-Bland <lbland@instructure.com>
Reviewed-by: Ryan Norton <rnorton@instructure.com>
Product-Review: Landon Gilbert-Bland <lbland@instructure.com>

app/models/discussion_topic.rb

@@ -204,7 +204,10 @@ class DiscussionTopic < ActiveRecord::Base
     return unless self.is_section_specific? ? @sections_changed : self.is_section_specific_before_last_save
     self.class.connection.after_transaction_commit do
       if self.context_module_tags.preload(:context_module).exists?
-        self.context_module_tags.map(&:context_module).uniq.each(&:invalidate_progressions)
+        self.context_module_tags.map(&:context_module).uniq.each do |cm|
+          cm.invalidate_progressions
+          cm.touch
+        end
       end
     end
   end

spec/models/discussion_topic_spec.rb

@@ -36,6 +36,7 @@ describe DiscussionTopic do
     opts.reverse_merge!({
       :workflow_state => "active"
     })
+    topic.sections_changed = true
     topic.is_section_specific = true
     topic.discussion_topic_section_visibilities <<
       DiscussionTopicSectionVisibility.new(
@@ -300,6 +301,18 @@ describe DiscussionTopic do
       expect(@topic.visible_for?(@student)).to be_truthy
     end
 
+    it 'should clear the context modules cache on section change' do
+      context_module = @course.context_modules.create!(name: 'some module')
+      context_module.add_item(type: 'discussion_topic', id: @topic.id)
+      context_module.updated_at = 1.day.ago
+      context_module.save!
+      last_updated_at = context_module.updated_at
+      add_section_to_topic(@topic, @course.course_sections.create!)
+      @topic.save!
+      context_module.reload
+      expect(last_updated_at).not_to eq context_module.updated_at
+    end
+
     it "should be visible to students when topic delayed_post_at is in the future" do
       @topic.delayed_post_at = 5.days.from_now
       @topic.save!