use multi-column plucks for preload_conversation_context_codes

faster than using hashes

Change-Id: I1f6006a650ae69d1f83293548d2571562212351f
Reviewed-on: https://gerrit.instructure.com/87883
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/models/user.rb

@@ -2142,15 +2142,12 @@ class User < ActiveRecord::Base
     end
   end
 
-  # don't instantiate a buttload of objects if we can help it
-  def self.pluck_global_id_rows(relation)
-    rows = ActiveRecord::Base.connection.select_all(relation.to_sql)
-    rows.each do |row|
-      row.each do |k, id|
-        row[k] = Shard.relative_id_for(id, Shard.current, Shard.birth)
+  def self.convert_global_id_rows(rows)
+    rows.map do |row|
+      row.map do |id|
+        Shard.relative_id_for(id, Shard.current, Shard.birth)
       end
     end
-    rows
   end
 
   def self.preload_conversation_context_codes(users)
@@ -2166,37 +2163,34 @@ class User < ActiveRecord::Base
     concluded_contexts = {}
 
     Shard.with_each_shard(shards.to_a) do
-      course_rows = pluck_global_id_rows(
+      course_rows = convert_global_id_rows(
           Enrollment.joins(:course).
               where(User.enrollment_conditions(:active)).
               where(user_id: users).
-              select([:user_id, :course_id]).
-              uniq)
-      course_rows.each do |r|
-        active_contexts[r['user_id']] ||= []
-        active_contexts[r['user_id']] << "course_#{r['course_id']}"
+              distinct.pluck(:user_id, :course_id))
+      course_rows.each do |user_id, course_id|
+        active_contexts[user_id] ||= []
+        active_contexts[user_id] << "course_#{course_id}"
       end
 
-      cc_rows = pluck_global_id_rows(
+      cc_rows = convert_global_id_rows(
           Enrollment.joins(:course).
               where(User.enrollment_conditions(:completed)).
               where(user_id: users).
-              select([:user_id, :course_id]).
-              uniq)
-      cc_rows.each do |r|
-        concluded_contexts[r['user_id']] ||= []
-        concluded_contexts[r['user_id']] << "course_#{r['course_id']}"
+              distinct.pluck(:user_id, :course_id))
+      cc_rows.each do |user_id, course_id|
+        concluded_contexts[user_id] ||= []
+        concluded_contexts[user_id] << "course_#{course_id}"
       end
 
-      group_rows = pluck_global_id_rows(
+      group_rows = convert_global_id_rows(
           GroupMembership.joins(:group).
               merge(User.instance_exec(&User.reflections[CANVAS_RAILS4_0 ? :current_group_memberships : 'current_group_memberships'].scope).only(:where)).
               where(user_id: users).
-              select([:user_id, :group_id]).
-              uniq)
-      group_rows.each do |r|
-        active_contexts[r['user_id']] ||= []
-        active_contexts[r['user_id']] << "group_#{r['group_id']}"
+              distinct.pluck(:user_id, :group_id))
+      group_rows.each do |user_id, group_id|
+        active_contexts[user_id] ||= []
+        active_contexts[user_id] << "group_#{group_id}"
       end
     end
     Shard.birth.activate do