From 0cdfe6f067e157ad1c9996d7cb687f405331c300 Mon Sep 17 00:00:00 2001
From: Codex Night Worker <codex-night@localhost>
Date: Fri, 13 Nov 2026 12:27:55 +0900
Subject: [PATCH] AI: address issue #437

---
 backend/app/models/post_implication.rb | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/backend/app/models/post_implication.rb b/backend/app/models/post_implication.rb
index 21c5a59..2ea8ec3 100644
--- a/backend/app/models/post_implication.rb
+++ b/backend/app/models/post_implication.rb
@@ -8,6 +8,7 @@ class PostImplication < ApplicationRecord
   validates :parent_post_id, presence: true
 
   validate :parent_post_mustnt_be_itself
+  validate :parent_post_mustnt_create_cycle
 
   private
 
@@ -16,4 +17,24 @@ class PostImplication < ApplicationRecord
       errors.add :parent_post_id, '親投稿に同じ投稿を設定することはできません．'
     end
   end
+
+  def parent_post_mustnt_create_cycle
+    return if post_id.blank? || parent_post_id.blank?
+    return if errors[:parent_post_id].present?
+
+    ancestor_ids = Set.new
+    frontier_ids = [parent_post_id]
+
+    until frontier_ids.empty?
+      if frontier_ids.include?(post_id)
+        errors.add :parent_post_id, :invalid
+        return
+      end
+
+      ancestor_ids.merge(frontier_ids)
+      frontier_ids = PostImplication.where(post_id: frontier_ids)
+          .distinct.pluck(:parent_post_id)
+          .reject { ancestor_ids.include?(_1) }
+    end
+  end
 end