Use join instead of subquery on Label.unprioritized scope

app/models/label.rb

@@ -39,7 +39,14 @@ class Label < ActiveRecord::Base
   end
 
   def self.unprioritized(project)
-    where.not(id: prioritized(project).select(:id))
+    labels = Label.arel_table
+    priorities = LabelPriority.arel_table
+
+    label_priorities = labels.join(priorities, Arel::Nodes::OuterJoin).
+                              on(labels[:id].eq(priorities[:label_id]).and(priorities[:project_id].eq(project.id))).
+                              join_sources
+
+    joins(label_priorities).where(priorities[:priority].eq(nil))
   end
 
   def self.left_join_priorities