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
+          if PostTag.kept.exists?(post_id: pt.post_id, tag_id: target_tag.id)
-            pt.update!(tag: target_tag)
-          rescue ActiveRecord::RecordNotUnique
             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

backend/spec/models/tag_spec.rb

@@ -0,0 +1,71 @@
+require 'rails_helper'
+
+RSpec.describe Tag, type: :model do
+  describe '.merge_tags' do
+    let!(:target_tag) { create(:tag) }
+    let!(:source_tag) { create(:tag) }
+
+    let!(:post_record) { Post.create!(url: 'https://example.com/posts/1', title: 'test post') }
+
+    context 'when merging a simple source tag' do
+      let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
+
+      it 'moves the post_tag, deletes the source tag, and aliases the source tag_name' do
+        described_class.merge_tags(target_tag, [source_tag])
+
+        expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
+        expect(Tag.exists?(source_tag.id)).to be(false)
+        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+      end
+    end
+
+    context 'when the target already has the same post_tag' do
+      let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
+      let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
+
+      it 'discards the duplicate source post_tag and keeps one active target post_tag' do
+        described_class.merge_tags(target_tag, [source_tag])
+
+        active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
+        discarded_source = PostTag.with_discarded.find(source_post_tag.id)
+
+        expect(active.count).to eq(1)
+        expect(discarded_source.discarded_at).to be_present
+        expect(Tag.exists?(source_tag.id)).to be(false)
+        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+      end
+    end
+
+    context 'when source_tags includes the target itself' do
+      let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
+
+      it 'ignores the target tag in source_tags' do
+        described_class.merge_tags(target_tag, [source_tag, target_tag])
+
+        expect(Tag.exists?(target_tag.id)).to be(true)
+        expect(Tag.exists?(source_tag.id)).to be(false)
+        expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
+      end
+    end
+
+    context 'when aliasing the source tag_name is invalid' do
+      let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
+      let!(:wiki_page) do
+        WikiPage.create!(
+            tag_name: source_tag.tag_name,
+            created_user: create_admin_user!,
+            updated_user: create_admin_user!)
+      end
+
+      it 'rolls back the transaction' do
+        expect {
+          described_class.merge_tags(target_tag, [source_tag])
+        }.to raise_error(ActiveRecord::RecordInvalid)
+
+        expect(Tag.exists?(source_tag.id)).to be(true)
+        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
+        expect(source_tag.tag_name.reload.canonical_id).to be_nil
+      end
+    end
+  end
+end

backend/spec/requests/tag_children_spec.rb

@@ -1,4 +1,3 @@
-# spec/requests/tag_children_spec.rb
 require "rails_helper"
 
 RSpec.describe "TagChildren", type: :request do