DEV: Ensure unique notification level per tag user (#28638)

TagUser.rb is used to set user notification levels for a tag, we don't have a unique index on the notification level itself. This means that there might be some weird case where a user may have multiple of the same notification level on a tag.

This PR adds a migration which de-duplicates this based on defaults, where we keep the earliest record in the event there is multiple notification level per-user-per-tag.

app/models/tag_user.rb

@@ -259,6 +259,7 @@ end
 #
 # Indexes
 #
-#  idx_tag_users_ix1  (user_id,tag_id,notification_level) UNIQUE
+#  index_tag_users_on_tag_id_and_user_id_and_notification_level  (tag_id,user_id,notification_level)
-#  idx_tag_users_ix2  (tag_id,user_id,notification_level) UNIQUE
+#  index_tag_users_on_user_id_and_tag_id                         (user_id,tag_id) UNIQUE
+#  index_tag_users_on_user_id_and_tag_id_and_notification_level  (user_id,tag_id,notification_level)
 #

db/migrate/20240829083823_ensure_unique_tag_user_notification_level.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class EnsureUniqueTagUserNotificationLevel < ActiveRecord::Migration[7.1]
+  def up
+    execute <<-SQL
+      DELETE FROM tag_users
+      USING tag_users AS dupe
+      WHERE tag_users.id > dupe.id
+      AND tag_users.user_id = dupe.user_id
+      AND tag_users.tag_id = dupe.tag_id;
+    SQL
+
+    add_index :tag_users, %i[user_id tag_id], unique: true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240829142639_remove_unique_constraint_from_tag_users_indexes.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+class RemoveUniqueConstraintFromTagUsersIndexes < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :tag_users, name: :idx_tag_users_ix1, algorithm: :concurrently
+    remove_index :tag_users, name: :idx_tag_users_ix2, algorithm: :concurrently
+
+    add_index :tag_users, %i[user_id tag_id notification_level], algorithm: :concurrently
+    add_index :tag_users, %i[tag_id user_id notification_level], algorithm: :concurrently
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

spec/fabricators/tag_user_fabricator.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+Fabricator(:tag_user) do
+  user
+  tag
+  notification_level { TagUser.notification_levels[:tracking] }
+end