feat: 投稿とタグのリレーション・テーブルについて論理削除と履歴を追加（#84） (#148)

#84 マイグレ修正

Merge remote-tracking branch 'origin/main' into feature/084

#84 構文エラー修正

#84

#84

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #148

backend/db/migrate/20251011200300_add_discard_to_post_tags.rb

@@ -0,0 +1,36 @@
+class AddDiscardToPostTags < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      DELETE
+        pt1
+      FROM
+        post_tags pt1
+        INNER JOIN
+          post_tags pt2
+          ON
+            pt1.post_id = pt2.post_id
+            AND pt1.tag_id = pt2.tag_id
+            AND pt1.id > pt2.id
+      ;
+    SQL
+
+    add_column :post_tags, :discarded_at, :datetime
+    add_index :post_tags, :discarded_at
+
+    add_column :post_tags, :is_active, :boolean,
+               as: 'discarded_at IS NULL', stored: true
+
+    add_column :post_tags, :active_unique_key, :string,
+               as: "CASE WHEN discarded_at IS NULL THEN CONCAT(post_id, ':', tag_id) ELSE NULL END",
+               stored: true
+
+    add_index :post_tags, :active_unique_key, unique: true, name: 'idx_post_tags_active_unique'
+
+    add_index :post_tags, [:post_id, :discarded_at]
+    add_index :post_tags, [:tag_id, :discarded_at]
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+end