Delegate :username rather than using a class method.

This is just a little cleaner?

app/models/post.rb

@@ -51,7 +51,9 @@ class Post < ActiveRecord::Base
   scope :public_posts, -> { joins(:topic).where('topics.archetype <> ?', Archetype.private_message) }
   scope :private_posts, -> { joins(:topic).where('topics.archetype = ?', Archetype.private_message) }
   scope :with_topic_subtype, ->(subtype) { joins(:topic).where('topics.subtype = ?', subtype) }
-
+  
+  delegate :username, to: :user
+  
   def self.hidden_reasons
     @hidden_reasons ||= Enum.new(:flag_threshold_reached, :flag_threshold_reached_again, :new_user_spam_threshold_reached)
   end
@@ -206,10 +208,6 @@ class Post < ActiveRecord::Base
     cooked
   end
 
-  def username
-    user.username
-  end
-
   def external_id
     "#{topic_id}/#{post_number}"
   end