#282

backend/app/models/tag.rb

@@ -150,15 +150,18 @@ class Tag < ApplicationRecord
         next if st == target_tag
 
         st.post_tags.find_each do |pt|
-          begin
-            pt.update!(tag: target_tag)
-          rescue ActiveRecord::RecordNotUnique
+          if PostTag.kept.exists?(post_id: pt.post_id, tag_id: target_tag.id)
             pt.discard_by!(nil)
+            # discard 後の update! は禁止なので DB を直に更新
+            pt.update_columns(tag_id: target_tag.id)
+          else
+            pt.update!(tag: target_tag)
           end
         end
 
         tag_name = st.tag_name
         st.destroy!
+        tag_name.reload
         tag_name.update!(canonical: target_tag.tag_name)
       end
     end