improve visiblity scope performance by EXIST

IN in combination with a subquery can lead to very poor performance if that subquery ends up being executed once for every candidate record. EXIST typically performs better and only rarely worse. Using a join would be even stable but this is harder to do in the visible scopes which end up being used in a lot of queries combined of various scopes.

app/models/notifications/scopes/visible.rb

@@ -36,9 +36,15 @@ module Notifications::Scopes
       # * the user has the permission to see the associated resource
       #
       # As currently only notifications for work packages exist, the implementation is work package specific.
+      # Using EXISTS as this performs better.
       def visible(user)
         recipient(user)
-          .where(resource_type: 'WorkPackage', resource_id: WorkPackage.visible(user).select(:id))
+          .where(resource_type: 'WorkPackage')
+          .where(WorkPackage
+                   .visible(user)
+                   .where("#{Notification.table_name}.resource_id = #{WorkPackage.table_name}.id")
+                   .arel
+                   .exists)
       end
     end
   end

app/models/work_package.rb

@@ -72,7 +72,12 @@ class WorkPackage < ApplicationRecord
   }
 
   scope :visible, ->(*args) {
-    where(project_id: Project.allowed_to(args.first || User.current, :view_work_packages))
+    # Using EXISTS as this performs better.
+    where(Project
+            .allowed_to(args.first || User.current, :view_work_packages)
+            .where("#{WorkPackage.table_name}.project_id = #{Project.table_name}.id")
+            .arel
+            .exists)
   }
 
   scope :in_status, ->(*args) do