Avoid the same foreign_key and counter_cache associations on SillyReply

`topic` and `reply` belongs_to associations on `SillyReply` are defined
with the same `foreign_key` (`parent_id`) and `counter_cache`
(`replies_count`) columns.
This would cause unintentional side-effect (e.g. saving `SillyReply`
object would cause double increment `replies_count`), so it is better to
avoid that side-effect.

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -827,6 +827,7 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
 
     silly = SillyReply.create(title: "gaga", content: "boo-boo")
     silly.reply = reply
+    silly.save!
 
     assert_equal 1, reply.reload[:replies_count]
     assert_equal 1, reply.replies.size

activerecord/test/cases/associations/cascaded_eager_loading_test.rb

@@ -117,9 +117,8 @@ class CascadedEagerLoadingTest < ActiveRecord::TestCase
   end
 
   def test_eager_association_loading_with_has_many_sti_and_subclasses
-    silly = SillyReply.new(title: "gaga", content: "boo-boo", parent_id: 1)
-    silly.parent_id = 1
-    assert silly.save
+    reply = Reply.new(title: "gaga", content: "boo-boo", parent_id: 1)
+    assert reply.save
 
     topics = Topic.all.merge!(includes: :replies, order: ["topics.id", "replies_topics.id"]).to_a
     assert_no_queries do

activerecord/test/models/reply.rb

@@ -9,6 +9,10 @@ class Reply < Topic
   has_many :silly_unique_replies, dependent: :destroy, foreign_key: "parent_id"
 end
 
+class SillyReply < Topic
+  belongs_to :reply, foreign_key: "parent_id", counter_cache: :replies_count
+end
+
 class UniqueReply < Reply
   belongs_to :topic, foreign_key: "parent_id", counter_cache: true
   validates_uniqueness_of :content, scope: "parent_id"
@@ -54,10 +58,6 @@ class WrongReply < Reply
   end
 end
 
-class SillyReply < Reply
-  belongs_to :reply, foreign_key: "parent_id", counter_cache: :replies_count
-end
-
 module Web
   class Reply < Web::Topic
     belongs_to :topic, foreign_key: "parent_id", counter_cache: true, class_name: "Web::Topic"