make ‘has_assignment_group’ cache key live longer

Change-Id: I8ee6f88e44f492b7471c1a6648339cac57c1886f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218102
Reviewed-by: Steven Burnett <sburnett@instructure.com>
Reviewed-by: James Williams <jamesw@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Tested-by: Jenkins
QA-Review: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

app/models/assignment_group.rb

@@ -57,6 +57,7 @@ class AssignmentGroup < ActiveRecord::Base
   after_save :update_student_grades
 
   before_destroy :destroy_scores
+  after_destroy :clear_context_has_assignment_group_cache
 
   def generate_default_values
     if self.name.blank?
@@ -164,6 +165,11 @@ class AssignmentGroup < ActiveRecord::Base
     true
   end
 
+  # this is just in case we happen to delete the last assignment_group in a course
+  def clear_context_has_assignment_group_cache
+    Rails.cache.delete(['has_assignment_group', global_context_id].cache_key) if context_id
+  end
+
   set_broadcast_policy do |p|
     p.dispatch :grade_weight_changed
     p.to { context.participating_students_by_date }

app/models/course.rb

@@ -1358,13 +1358,14 @@ class Course < ActiveRecord::Base
 
   def require_assignment_group
     shard.activate do
-      return if Rails.cache.read(['has_assignment_group', self].cache_key)
+      key = ['has_assignment_group', self.global_id].cache_key
+      return if Rails.cache.read(key)
       if self.assignment_groups.active.empty?
         Shackles.activate(:master) do
           self.assignment_groups.create!(name: t('#assignment_group.default_name', "Assignments"))
         end
       end
-      Rails.cache.write(['has_assignment_group', self].cache_key, true)
+      Rails.cache.write(key, true)
     end
   end