diff --git a/activerecord/test/models/comment.rb b/activerecord/test/models/comment.rb
index 61c54a77a7..740aa593ac 100644
--- a/activerecord/test/models/comment.rb
+++ b/activerecord/test/models/comment.rb
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+# `counter_cache` requires association class before `attr_readonly`.
+class Post < ActiveRecord::Base; end
+
 class Comment < ActiveRecord::Base
   scope :limit_by, lambda { |l| limit(l) }
   scope :containing_the_letter_e, -> { where("comments.body LIKE '%e%'") }