Prefer polymorphism over `is_a?`

app/controllers/concerns/notes_actions.rb

@@ -18,7 +18,7 @@ module NotesActions
     @notes = prepare_notes_for_rendering(@notes)
 
     notes_json[:notes] =
-      if noteable.is_a?(Issue)
+      if noteable.discussions_rendered_on_frontend?
         note_serializer.represent(@notes)
       else
         @notes.map { |note| note_json(note) }
@@ -87,7 +87,7 @@ module NotesActions
     if note.persisted?
       attrs[:valid] = true
 
-      if noteable.is_a?(Issue)
+      if noteable.discussions_rendered_on_frontend?
         attrs.merge!(note_serializer.represent(note))
       else
         attrs.merge!(

app/models/award_emoji.rb

@@ -37,7 +37,7 @@ class AwardEmoji < ActiveRecord::Base
   end
 
   def expire_etag_cache
-    return unless awardable.is_a?(Note)
+    return unless awardable.respond_to?(:expire_etag_cache)
 
     awardable.expire_etag_cache
   end

app/models/concerns/noteable.rb

@@ -24,6 +24,10 @@ module Noteable
     DiscussionNote::NOTEABLE_TYPES.include?(base_class_name)
   end
 
+  def discussions_rendered_on_frontend?
+    false
+  end
+
   def discussion_notes
     notes
   end

app/models/issue.rb

@@ -269,6 +269,10 @@ class Issue < ActiveRecord::Base
     end
   end
 
+  def discussions_rendered_on_frontend?
+    true
+  end
+
   private
 
   # Returns `true` if the given User can read the current Issue.

app/models/note.rb

@@ -300,12 +300,12 @@ class Note < ActiveRecord::Base
   end
 
   def expire_etag_cache
-    return unless for_issue?
+    return unless noteable.discussions_rendered_on_frontend?
 
     key = Gitlab::Routing.url_helpers.project_noteable_notes_path(
-      noteable.project,
+      project,
       target_type: noteable_type.underscore,
-      target_id: noteable.id
+      target_id: noteable_id
     )
     Gitlab::EtagCaching::Store.new.touch(key)
   end