#281

backend/app/models/tag.rb

@@ -162,14 +162,18 @@ class Tag < ApplicationRecord
         end
 
         source_tag_name = source_tag.tag_name
-        nico_flg = source_tag.nico?
+
+        if source_tag_name.wiki_page.present?
+          raise ActiveRecord::RecordInvalid.new(source_tag_name)
+        end
+
         source_tag.discard!
-        source_tag_name.reload
 
-        if nico_flg
-          source_tag_name.destroy!
+        if source_tag.nico?
+          source_tag_name.discard!
         else
-          source_tag_name.update!(canonical: target_tag.tag_name)
+          source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
+                                         updated_at: Time.current)
         end
       end
 

backend/app/models/tag_name.rb

@@ -1,4 +1,8 @@
 class TagName < ApplicationRecord
+  include Discard::Model
+
+  default_scope -> { kept }
+
   has_one :tag
   has_one :wiki_page
 

backend/app/models/tag_name_sanitisation_rule.rb

@@ -24,12 +24,15 @@ class TagNameSanitisationRule < ApplicationRecord
             existing_tn = existing_tn.canonical || existing_tn
             next if existing_tn.id == tn.id
 
-            if existing_tn.tag
-              Tag.merge_tags!(existing_tn.tag, tn.tag) if tn.tag
-            elsif tn.tag
-              tn.tag.update_columns(tag_name_id: existing_tn.id, updated_at: Time.current)
+            existing_tag = Tag.find_by(tag_name_id: existing_tn.id)
+            source_tag = Tag.find_by(tag_name_id: tn.id)
+
+            if existing_tag
+              Tag.merge_tags!(existing_tag, source_tag) if tn.tag
+            elsif source_tag
+              source_tag.update_columns(tag_name_id: existing_tn.id, updated_at: Time.current)
             end
-            tn.destroy!
+            tn.discard!
 
             next
           end

backend/app/models/wiki_page.rb

@@ -2,6 +2,10 @@ require 'set'
 
 
 class WikiPage < ApplicationRecord
+  include Discard::Model
+
+  default_scope -> { kept }
+
   has_many :wiki_revisions, dependent: :destroy
   belongs_to :created_user, class_name: 'User'
   belongs_to :updated_user, class_name: 'User'

backend/db/migrate/20260311232100_add_discarded_at_to_tag_names.rb

@@ -0,0 +1,6 @@
+class AddDiscardedAtToTagNames < ActiveRecord::Migration[8.0]
+  def change
+    add_column :tag_names, :discarded_at, :datetime
+    add_index :tag_names, :discarded_at
+  end
+end

backend/db/migrate/20260311232300_add_discarded_at_to_wiki_pages.rb

@@ -0,0 +1,6 @@
+class AddDiscardedAtToWikiPages < ActiveRecord::Migration[8.0]
+  def change
+    add_column :wiki_pages, :discarded_at, :datetime
+    add_index :wiki_pages, :discarded_at
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_03_11_123100) do
+ActiveRecord::Schema[8.0].define(version: 2026_03_11_232300) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -142,7 +142,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_11_123100) do
     t.bigint "canonical_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
     t.index ["canonical_id"], name: "index_tag_names_on_canonical_id"
+    t.index ["discarded_at"], name: "index_tag_names_on_discarded_at"
     t.index ["name"], name: "index_tag_names_on_name", unique: true
   end
 
@@ -204,7 +206,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_11_123100) do
     t.bigint "updated_user_id", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
     t.index ["created_user_id"], name: "index_wiki_pages_on_created_user_id"
+    t.index ["discarded_at"], name: "index_wiki_pages_on_discarded_at"
     t.index ["tag_name_id"], name: "index_wiki_pages_on_tag_name_id", unique: true
     t.index ["updated_user_id"], name: "index_wiki_pages_on_updated_user_id"
   end

backend/spec/models/tag_name_sanitisation_rule_spec.rb

@@ -69,7 +69,8 @@ RSpec.describe TagNameSanitisationRule, type: :model do
 
       it 'moves the tag to the existing tag_name' do
         described_class.apply!
-        expect(source_tag.reload.tag_name_id).to eq(existing.id)
+        expected_tag_name_id = existing.canonical_id || existing.id
+        expect(source_tag.reload.tag_name_id).to eq(expected_tag_name_id)
         expect(TagName.exists?(source_tag_name_id)).to be(false)
       end
     end

backend/spec/models/tag_spec.rb

@@ -13,14 +13,9 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the source post_tag, creates an active target post_tag, ' +
-         'discards the source tag, and aliases the source tag_name' do
+      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
-        expect(target_tag.reload.post_count).to eq(1)
-
         source_pt = PostTag.with_discarded.find(source_post_tag.id)
         active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
@@ -29,7 +24,9 @@ RSpec.describe Tag, type: :model do
         expect(active_target).to be_present
 
         expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
-        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
@@ -37,8 +34,7 @@ RSpec.describe Tag, type: :model 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 source post_tag, keeps one active target post_tag, ' +
-         'and aliases the source tag_name' do
+      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
         source_pt = PostTag.with_discarded.find(source_post_tag.id)
@@ -47,17 +43,12 @@ RSpec.describe Tag, type: :model do
         expect(source_pt.discarded_at).to be_present
         expect(source_pt.tag_id).to eq(source_tag.id)
         expect(active.count).to eq(1)
-
-        expect(source_pt.discarded_at).to be_present
-        expect(source_pt.tag_id).to eq(source_tag.id)
-        expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
-        expect(target_tag.reload.post_count).to eq(1)
-
         expect(active.first.id).to eq(target_post_tag.id)
 
         expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
-        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
@@ -67,10 +58,6 @@ RSpec.describe Tag, type: :model do
       it 'ignores the target in source_tags while still merging the source tag' 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(target_tag.reload.post_count).to eq(1)
-
         source_pt = PostTag.with_discarded.find(source_post_tag.id)
         active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
@@ -79,23 +66,65 @@ RSpec.describe Tag, type: :model do
         expect(source_pt.discarded_at).to be_present
         expect(source_pt.tag_id).to eq(source_tag.id)
         expect(active_target).to be_present
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(target_tag.reload.post_count).to eq(1)
+      end
+    end
+
+    context 'when the source tag_name is invalid under sanitisation rules' do
+      let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
+      let!(:sanitisation_rule) do
+        TagNameSanitisationRule.create!(
+          priority: 99_999,
+          source_pattern: 'INVALIDTOKEN',
+          replacement: ''
+        )
+      end
+
+      before do
+        source_tag_name.update_columns(
+          name: "#{ source_tag_name.name }INVALIDTOKEN",
+          updated_at: Time.current
+        )
+      end
+
+      it 'still merges, but discards the source tag_name instead of aliasing it' do
+        described_class.merge_tags!(target_tag, [source_tag])
+
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name.id)
+
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
+
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
-    context 'when aliasing the source tag_name would be invalid under normal validation' do
+    context 'when the source tag_name has a wiki_page' 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,
+          tag_name: source_tag_name,
           created_user: create_admin_user!,
           updated_user: create_admin_user!
         )
       end
 
-      it 'still merges by bypassing validations' do
+      it 'rolls back the transaction' do
         expect {
           described_class.merge_tags!(target_tag, [source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
+
+        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
+        expect(PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)).to be_nil
+        expect(source_tag_name.reload.canonical_id).to be_nil
+        expect(target_tag.reload.post_count).to eq(0)
       end
     end
 
@@ -104,16 +133,16 @@ RSpec.describe Tag, type: :model do
       let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
       let!(:source_tag_name_id) { source_tag.tag_name_id }
 
-      it 'deletes the source tag_name instead of aliasing it' do
+      it 'discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(TagName.exists?(source_tag_name_id)).to be(false)
-        expect(target_tag.reload.post_count).to eq(0)
+        discarded_source_tag = Tag.with_discarded.find(source_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name_id)
 
-        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
-        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
-        expect(source_tag.tag_name.reload.canonical_id).to be_nil
+        expect(discarded_source_tag).to be_discarded
+        expect(discarded_source_tag_name).to be_discarded
+        expect(discarded_source_tag_name.canonical_id).to be_nil
+        expect(target_tag.reload.post_count).to eq(0)
       end
     end
   end