improve performance of needs_grading_count in course_json

test plan:
* regression test 'needs_grading_count' includes in
 course index api calls

closes #CNVS-21156

Change-Id: Ic20704ec3704ed9e8230ad424cad2e5e92fccb68
Reviewed-on: https://gerrit.instructure.com/56465
Tested-by: Jenkins
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/models/assignments/needs_grading_count_query.rb

@@ -1,10 +1,40 @@
 module Assignments
   class NeedsGradingCountQuery
-    attr_reader :assignment, :user
 
-    def initialize(_assignment, _user)
+    # holds values so we don't have to recompute them over and over again
+    class CourseProxy
+      attr_reader :course, :user
+
+      def initialize(_course, _user)
+        @course = _course
+        @user = _user
+      end
+
+      def da_enabled?
+        @da_enabled ||= course.feature_enabled?(:differentiated_assignments)
+      end
+
+      def section_visibilities
+        @section_visibilities ||= course.section_visibilities_for(user)
+      end
+
+      def visibility_level
+        @visibility_level ||= course.enrollment_visibility_level_for(user, section_visibilities)
+      end
+
+      def visible_section_ids
+        @visible_section_ids ||= section_visibilities.map{|v| v[:course_section_id]}
+      end
+    end
+
+    attr_reader :assignment, :user, :course_proxy
+
+    delegate :course, :da_enabled?, :section_visibilities, :visibility_level, :visible_section_ids, :to => :course_proxy
+
+    def initialize(_assignment, _user, _course_proxy=nil)
       @assignment = _assignment
       @user = _user
+      @course_proxy = _course_proxy || CourseProxy.new(@assignment.context, @user)
     end
 
     def count
@@ -76,25 +106,5 @@ module Assignments
     def joined_submissions
       assignment.submissions.joins("INNER JOIN enrollments e ON e.user_id = submissions.user_id")
     end
-
-    def visible_section_ids
-      section_visibilities.map{|v| v[:course_section_id]}
-    end
-
-    def visibility_level
-      course.enrollment_visibility_level_for(user, section_visibilities)
-    end
-
-    def section_visibilities
-      course.section_visibilities_for(user)
-    end
-
-    def course
-      assignment.context
-    end
-
-    def da_enabled?
-      course.feature_enabled?(:differentiated_assignments)
-    end
   end
 end

lib/api/v1/course_json.rb

@@ -71,7 +71,8 @@ module Api::V1
 
     def needs_grading_count(enrollments, course)
       if include_grading && enrollments && enrollments.any? { |e| e.participating_instructor? }
-        course.assignments.active.to_a.sum{|a| Assignments::NeedsGradingCountQuery.new(a, user).count }
+        proxy = Assignments::NeedsGradingCountQuery::CourseProxy.new(course, user)
+        course.assignments.active.to_a.sum{|a| Assignments::NeedsGradingCountQuery.new(a, user, proxy).count }
       end
     end