improve section_context_codes performance

don't initialize AR records just to get asset_strings

refs #CNVS-21317

Change-Id: If6afb664807926936d6957cac6aed24cfadc74b4
Reviewed-on: https://gerrit.instructure.com/61251
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/controllers/users_controller.rb

@@ -530,7 +530,7 @@ class UsersController < ApplicationController
     Shackles.activate(:slave) do
       prepare_current_user_dashboard_items
 
-      if @show_recent_feedback = (@current_user.student_enrollments.active.present?)
+      if @show_recent_feedback = (@current_user.student_enrollments.active.exists?)
         @recent_feedback = (@current_user && @current_user.recent_feedback) || []
       end
     end

app/models/course.rb

@@ -2093,21 +2093,32 @@ class Course < ActiveRecord::Base
     end
   end
 
-  def sections_visible_to(user, sections = active_course_sections)
+  # returns :all, :none, or an array of section ids
+  def course_section_visibility(user)
     visibilities = section_visibilities_for(user)
     visibility = enrollment_visibility_level_for(user, visibilities)
-    section_ids = visibilities.map{ |s| s[:course_section_id] }
-    is_scope = sections.respond_to?(:where)
-
     if [:full, :limited, :restricted, :sections].include?(visibility)
       if visibility == :sections || visibilities.all?{ |v| ['StudentEnrollment', 'StudentViewEnrollment', 'ObserverEnrollment'].include? v[:type] }
-        is_scope ? sections.where(:id => section_ids) : sections.select{|section| section_ids.include?(section.id)}
+        visibilities.map{ |s| s[:course_section_id] }
       else
-        sections
+        :all
       end
     else
+      :none
+    end
+  end
+
+  def sections_visible_to(user, sections = active_course_sections)
+    is_scope = sections.respond_to?(:where)
+    section_ids = course_section_visibility(user)
+    case section_ids
+    when :all
+      sections
+    when :none
       # return an empty set, but keep it as a scope for downstream consistency
       is_scope ? sections.none : []
+    when Array
+      is_scope ? sections.where(:id => section_ids) : sections.select{|section| section_ids.include?(section.id)}
     end
   end
 

app/models/user.rb

@@ -1806,7 +1806,7 @@ class User < ActiveRecord::Base
             order('last_updated_at_from_db DESC').
             limit(opts[:limit]).to_a
 
-          submissions = submissions.sort_by{|t| (t.last_updated_at_from_db.to_datetime.in_time_zone rescue nil) || t.created_at}.reverse
+          submissions = submissions.sort_by{|t| t['last_updated_at_from_db'] || t.created_at}.reverse
           submissions = submissions.uniq
           submissions.first(opts[:limit])
 
@@ -2139,9 +2139,27 @@ class User < ActiveRecord::Base
   def section_context_codes(context_codes)
     course_ids = context_codes.grep(/\Acourse_\d+\z/).map{ |s| s.sub(/\Acourse_/, '').to_i }
     return [] unless course_ids.present?
-    Course.where(id: course_ids).inject([]) do |ary, course|
-      ary.concat course.sections_visible_to(self).map(&:asset_string)
+
+    section_ids = []
+    full_course_ids = []
+    Course.where(id: course_ids).each do |course|
+      result = course.course_section_visibility(self)
+      case result
+      when Array
+        section_ids.concat(result)
+      when :all
+        full_course_ids << course.id
+      end
     end
+
+    if full_course_ids.any?
+      current_shard = Shard.current
+      Shard.partition_by_shard(full_course_ids) do |shard_course_ids|
+        section_ids.concat(CourseSection.active.where(:course_id => shard_course_ids).pluck(:id).
+            map{|id| Shard.relative_id_for(id, Shard.current, current_shard)})
+      end
+    end
+    section_ids.map{|id| "course_section_#{id}"}
   end
 
   def manageable_courses(include_concluded = false)